Problem 1 (C++)

#include <iostream>
using namespace std;

int main()
{ 
        int sum = 0;
        int i1 = 999/5;
        int i2 = 999/3;
        int i3 = 999/15;

        for(int i=1; i <= i1; i++)
        {
                sum += 5*i;
        }
        for(int i=1; i <= i2; i++)
        {
                sum += 3*i;
        }
        for(int i=1; i <= i3; i++)
        {
                sum -= 15*i;
        }

        cout <<sum;
        cin >>sum;//pause
        return 0;
}