#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <math.h>


int main()
{
	double p1_x,p1_y;
	double p2_x,p2_y;
	double x,y;
	double wk;
	double calc_f;

	scanf("%lf %lf %lf %lf",&p1_x,&p1_y,&p2_x,&p2_y);
	x = fabs(p1_x-p2_x);
	y = fabs(p1_y-p2_y);
	
	calc_f = (pow(x,2) + pow(y,2));
	calc_f = sqrt(calc_f);
	printf("%lf\n",calc_f);

	return 0;
}