I am doing a code and in that there is a line where a,b,c,d are integers,

      int a ,b ,c ,d;
      long long sum =a + b +c+ d;

I got overflow as error
After i executed code i h=got overflow but after referring answer the sum need to be written as

     long long sum=a;
     sum+=b;
     sum+=c;
     sum+=d;

For this line i got correct result .But why I am unable to understand the difference?