Program to quickly sum integers from a start to an end.

sum.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv) {
  // if at least 2 arguments are given to the program
  if (argc > 2) {
    // first integer
    long long f = atoll(argv[1]);
    // last integer
    long long l = atoll(argv[2]);
    printf("%lli\n", (long long)((((l-f)+1)/2.0)*(f+l)));
  }
  // if 1 arguments is given to the program
  else if (argc == 2) {
    // last integer
    long long l = atoll(argv[1]);
    printf("%lli\n", (l/2)*(1+l));
  }
}