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

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

	scanf("%ld %ld %ld %ld", &x1,&y1,&x2,&y2);
	distance = sqrt(pow((x2-x1),2) + pow((y2-y1),2));

	printf("%f\n",distance);
	return 0;
}