#include <iostream>
#include <cmath>

using namespace std;

typedef struct
{
	double x;
	double y;
} Point;

double distance(const Point& a, const Point& b)
{
	return sqrt((a.x - b.x)*(a.x - b.x) + (a.y - b.y)*(a.y - b.y));
}

int main()
{
	Point a, b;

	cin >> a.x >> a.y >> b.x >> b.y;

	cout << distance(a, b) << endl;

	return 0;
}