#include<iostream>
#include<cmath>
#include<iomanip>

using namespace std;

int main(){
    double x1,y1,x2,y2;
    if(!(cin>>x1>>y1>>x2>>y2))
    return 0;
    
    if(x1<-50||x1>50)||y1<-50||y1>50)||x2<-50||x2>50)||y2<-50||y2>50){
        cout<<"Invalid input"<<endl;
    }else{
        double dist=sqrt(pow(x2-x1,2)+pow(y2-y1,2));
        cout<<fixed<<setprecision(2)<<dist<<endl;
    }
    return 0;
}