http://projecteuler.net/problem=1

#include <iostream>

using namespace std;

int SumOfArithmeticSequence(int nEqualDiff, int nTopLimit)
{
    int nCount = nTopLimit / nEqualDiff;
    int nSum = nEqualDiff * nCount * (nCount + 1) / 2;
    return nSum;
}

int main()
{
    int nResult = SumOfArithmeticSequence(3, 999) + SumOfArithmeticSequence(5, 999) - SumOfArithmeticSequence(15, 999);
    cout << nResult << endl;
    return 0;
}