// p03_03.cpp Sum of the 1st, 3rd, 6th, 10t, 15th, ... elements

// of a number sequence entered by the user.

#include <iostream>

using namespace std;



int main() {

float x, s =0;

int i = 0, iAdd = 1, diff = 1;

cout << "Add numbers followed by a nonnumeric character: \n";

while (cin >> x) {

 if (++i == iAdd) {

  s += x;

  iAdd += ++diff;

 }

}

cout << "Sum of 1st 3rd 6th 10th 15th ... numbers is " << s << endl;

return 0;

}