---

#include <iostream>
#include <cmath>
using namespace std;

double distance(double x1, double y1, double x2, double y2) {
    double dx = x2 - x1;
    double dy = y2 - y1;

    return sqrt(dx * dx + dy * dy);
}

int main() {
    double x1, y1, x2, y2;

    cin >> x1 >> y1 >> x2 >> y2;

    cout << distance(x1, y1, x2, y2);

    return 0;
}