#include <bits/stdc++.h>
using namespace std;
inline void boost() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}
inline void solve() {
  boost();
  int r, x1, y1, x2, y2;
  cin >> r >> x1 >> y1 >> x2 >> y2;
  cout << sqrt(abs(x1 - x2) * abs(x1 - x2) + abs(y1 - y2) * abs(y1 - y2)) /
              (2 * r);
}
int main() {
  int kolT = 1;
  while (kolT) {
    solve();
    kolT--;
  }
  exit(0);
}
