my df
sp state year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0 cm 2020 0.01 0.023 0.015 0.021 0.0235 0.026 0.0285 0.031 0.0335 0.036 0.0385 0.041 0.0435 0.046 0.0485 0.051 0.0535 0.056
1 cm 2020 null 0.32 0.5 0.68 0.86 1.04 1.22 1.4 1.58 1.76 1.94 2.12 2.3 2.48 2.66 2.84 3.02 3.2
2 cm 2020 null null 0.6 1.339 1.6965 2.054 2.4115 2.769 3.1265 3.484 3.8415 4.199 4.5565 4.914 5.2715 5.629 5.9865 6.344
3 cm 2020 null null null 1.998 2.533 3.068 3.603 4.138 4.673 5.208 5.743 6.278 6.813 7.348 7.883 8.418 8.953 9.488
4 cm 2020 null null null null 3.3695 4.082 4.7945 5.507 6.2195 6.932 7.6445 8.357 9.0695 9.782 10.4945 11.207 11.9195 12.632
5 cm 2020 null null null null null 5.096 5.986 6.876 7.766 8.656 9.546 10.436 11.326 12.216 13.106 13.996 14.886 15.776
6 cm 2020 null null null null null null 7.1775 8.245 9.3125 10.38 11.4475 12.515 13.5825 14.65 15.7175 16.785 17.8525 18.92
7 cm 2020 null null null null null null null 9.614 10.859 12.104 13.349 14.594 15.839 17.084 18.329 19.574 20.819 22.064
8 cm 2020 null null null null null null null null 12.4055 13.828 15.2505 16.673 18.0955 19.518 20.9405 22.363 23.7855 25.208
9 cm 2020 null null null null null null null null null 15.552 17.152 18.752 20.352 21.952 23.552 25.152 26.752 28.352
10 cm 2020 null null null null null null null null null null 19.0535 20.831 22.6085 24.386 26.1635 27.941 29.7185 31.496
11 cm 2020 null null null null null null null null null null null 22.91 24.865 26.82 28.775 30.73 32.685 34.64
12 cm 2020 null null null null null null null null null null null null 27.1215 29.254 31.3865 33.519 35.6515 37.784
13 cm 2020 null null null null null null null null null null null null null 31.688 33.998 36.308 38.618 40.928
14 cm 2020 null null null null null null null null null null null null null null 36.6095 39.097 41.5845 44.072
15 cm 2020 null null null null null null null null null null null null null null null 41.886 44.551 47.216
16 cm 2020 null null null null null null null null null null null null null null null null 47.5175 50.36
17 cm 2020 null null null null null null null null null null null null null null null null null 53.504
0 mp 2021 0.01 0.023 0.015 0.021 0.0235 0.026 0.0285 0.031 0.0335 0.036 0.0385 0.041 0.0435 0.046 0.0485 0.051 0.0535 0.056
1 mp 2021 null 0.32 0.5 0.68 0.86 1.04 1.22 1.4 1.58 1.76 1.94 2.12 2.3 2.48 2.66 2.84 3.02 3.2
2 mp 2021 null null 0.6 1.339 1.6965 2.054 2.4115 2.769 3.1265 3.484 3.8415 4.199 4.5565 4.914 5.2715 5.629 5.9865 6.344
3 mp 2021 null null null 1.998 2.533 3.068 3.603 4.138 4.673 5.208 5.743 6.278 6.813 7.348 7.883 8.418 8.953 9.488
4 mp 2021 null null null null 3.3695 4.082 4.7945 5.507 6.2195 6.932 7.6445 8.357 9.0695 9.782 10.4945 11.207 11.9195 12.632
5 mp 2021 null null null null null 5.096 5.986 6.876 7.766 8.656 9.546 10.436 11.326 12.216 13.106 13.996 14.886 15.776
6 mp 2021 null null null null null null 7.1775 8.245 9.3125 10.38 11.4475 12.515 13.5825 14.65 15.7175 16.785 17.8525 18.92
7 mp 2021 null null null null null null null 9.614 10.859 12.104 13.349 14.594 15.839 17.084 18.329 19.574 20.819 22.064
8 mp 2021 null null null null null null null null 12.4055 13.828 15.2505 16.673 18.0955 19.518 20.9405 22.363 23.7855 25.208
9 mp 2021 null null null null null null null null null 15.552 17.152 18.752 20.352 21.952 23.552 25.152 26.752 28.352
10 mp 2021 null null null null null null null null null null 19.0535 20.831 22.6085 24.386 26.1635 27.941 29.7185 31.496
11 mp 2021 null null null null null null null null null null null 22.91 24.865 26.82 28.775 30.73 32.685 34.64
12 mp 2021 null null null null null null null null null null null null 27.1215 29.254 31.3865 33.519 35.6515 37.784
13 mp 2021 null null null null null null null null null null null null null 31.688 33.998 36.308 38.618 40.928
14 mp 2021 null null null null null null null null null null null null null null 36.6095 39.097 41.5845 44.072
15 mp 2021 null null null null null null null null null null null null null null null 41.886 44.551 47.216
16 mp 2021 null null null null null null null null null null null null null null null null 47.5175 50.36
17 mp 2021 null null null null null null null null null null null null null null null null null 53.504

Please help with python pandas way

I want to calculate the average in a semi diagnal way based on sp, state year.
Example:
for sp-0, state-cm and year -2020
average(D2,E3,F4,G5,H6,I7,J8,K9,L10,M11,N12,O13,P14,Q15,R16,S17,T18,U19)
for sp-1,state-cm and yeaer – 2020
average(E2,F3……..T17,U18)