I’m a beginner CSE student and I have a question about how the return statement works.

For the code:

#include <stdio.h>

int main()
{
    int a,b;
    printf("Enter a's Value n");
    scanf("%d",&a);
    printf("Enter b's Value n");
    scanf("%d",&b);
    printf("Sum of numbers is %d",a+b);
}

The output is:

Enter a's Value
23
Enter b's Value
62
Sum of numbers is 85
--------------------------------
Process exited after 2.972 seconds with return value 20
Press any key to continue . . .

However, when I add a return 0; at the end of the function, the output is:

Enter a's Value
23
Enter b's Value
62
Sum of numbers is 85
--------------------------------
Process exited after 4.276 seconds with return value 0
Press any key to continue . . .

Why is the return value 20 in the first code, despite me not passing a return value??