Click here to download sum.cpp.

#include <iostream>
using std::cin;
using std::cout;

const char* strAns1 = "The sum of all integers up to ";
const char* strAns2 = " is ";
const char* strAns3 = "\n";

int sum(int a) {
  if (a < 1)
    return 0;
  else
    return a + sum(a-1);
}

int main() {
  int n, s;
  cout << "Enter an integer: ";
  cin >> n;
  s = sum(n);
  cout << "The sum of all integers up to " << n << " is " << s << "\n";
  return 0;
}