#include <stdio.h>
#include <math.h>

	double cal_distance(double x1,double y1,double x2,double y2)
	{

		double X,Y;
		double D,d;

		X = x2 - x1;
		Y = y2 - y1;

		D = (X*X)+(Y*Y);

		d = sqrt(D);
		
		return d ;
    }

int main(void)
{

	int  x1, x2, y1, y2;
	double distance;
	
 	scanf("%d %d %d %d",&x1,&y1,&x2,&y2);
	
	distance = cal_distance(x1,y1,x2,y2);
	
	printf("%f\n",distance);

	return 0;

}