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

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

	scanf("%f%f%f%f", &x1,&y1,&x2,&y2);
	dis = (x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
	printf("%f\n", sqrt(dist));

	return 0;
}