#include<iostream>
#include<cstdio>
using namespace std;

int main() {
    double a, b, x, y;
    while (cin >> a && cin >> b && cin >> x && cin >> y) {
        double ans = sqrt((x - a)*(x - a) + (y - b)*(y - b));
        printf("%.2f\n", ans);
    }
    ;
}