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

int main(void){
	double x1, y1, x2, y2, d;
	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
	d = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
	printf("%f\n", d);
	return 0;
}