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

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

	cout << "input numbers (x1 y1 x2 y2)>>";
	cin >> x1 >> y1 >> x2 >> y2;

	cout << sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2)) << endl;
}