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

using namespace std;

int main()
{
    double x1, y1, x2, y2;
    cin >> x1 >> y1 >> x2 >> y2;
    double dis;
    dis = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
    printf("%.4lf",dis);
    return 0;
}