C/stdlib.h/atoll
atoll: converts *str into a long long int value
//Declaration: long long int atoll(const char *str);
#include <stdlib.h>
#include <stdio.h>
int main(void)
{
char num1[80], num2[80];
printf("Enter first: ");
gets(num1);
printf("Enter second: ");
gets(num2);
printf("The sum is: %ld.", atoll(num1)+atoll(num2));
return 0;
}
/*
Enter first: 1
Enter second: 2
The sum is: 3.
*/