#include <bits/stdc++.h>
using namespace std;
int main(){
  cout << fixed << setprecision(20);
  double x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  cout << sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2));
}
