#include<iostream>
#include<math.h>
#include<stdio.h>
using namespace std;
int main(){
    double x1,y1,x2,y2;
    cin>>x1>>y1>>x2>>y2;
    printf("%.8f\n",sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2)));
    return 0;
}
