#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int main(){
    double px1, py1;
    double px2, py2;
    cin >> px1 >> py1 >> px2 >> py2;
    printf( "%.4lf", sqrt(pow(px1-px2, 2) + pow(py1-py2, 2)) );
    return 0;
}