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

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