#include <iostream>
using namespace std;

int main()
{
   long i, n;
cin >> n;

if ((n >= 1) && (n <= 10000))
cout << "sum=" << n *(n + 1) / 2;

else if ((n <= 1) && (n >= (-10000)))
cout << "sum=" << (-((-n) *(1 -n) / 2) + 1);

cout << endl;
return 0;
}