#include<bits/stdc++.h>
using namespace std;
int main(){
    double xa,ya,xb,yb;
    scanf("%lf%lf%lf%lf",&xa,&ya,&xb,&yb);
    printf("%.3lf",sqrt((xa-ya)*(xa-ya)+(xb-yb)*(xb-yb)));
    return 0;
}