#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int main()
{
    double x1, x2, y1, y2, ans;
    cin >> x1 >> y1 >> x2 >> y2;
    ans = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
    printf("%.4lf", ans);

    return 0;
}