#include <bits/stdc++.h>
using namespace std;

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

  cin >> x1 >> y1 >> x2 >> y2;
  cout << fixed << setprecision(10) << sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)) << endl;

  return (0);
}
#include<stdio.h>

int main()
{
	int x,y,z;
	scanf("%d %d %d",&x,&y,&z);
	if (x>9 || x<1 || y>9 || y<1 || z>9 || z<1)
	{
		printf("Error!");
	}
	else if( x == y && y == z && x == z){
		printf("No");
	}
	else if (x == z || x == y || y == z){
		printf("Yes");
	}
	else if (x>y || z>x || y>z){
		printf("No");
	}	
}