// total mass of N balls
#include <iostream>
#include <math.h>
using namespace std;

const int N = 100; // no. of balls

int main()
{
  double M=0;      // partial sum, which becomes total mass

  for (int i=0; i<N; ++i){   // repeat N times
    double m  = i; // m_i (mass of i-th ball)
    M += m ;       // i.e. M=M+m ( add m_i to partial sum )
  }
  cout << M << endl;
}