//Aideen's Code for P22 {aideen[at]gmail[dot]com}
#include <iostream>
using namespace std;

int x, y;

int main() {
  cin >> x >> y;

  if (x<y || x>=2*y)
    cout << "No Number" << endl;
  else 
    cout << y*(y-1)/2 + x-y+1 << endl; // below points + this row
  return 0;
}