#include<stdio.h>
#include<iostream>
#include<math.h>
using namespace std;
int main()
{
    double a,b,c,d;
    cin >> a >> b >> c >> d;
    double a1,a2;
    a1 = fabs(c-a);
    a2 = fabs(d-b);
    double a3 = sqrt(a1*a1+a2*a2);
    printf("%.3lf",a3);
}