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

int main(void)          
{
	double xone, yone, xtwo, ytwo;

	scanf("%lf %lf %lf %lf", &xone, &yone, &xtwo, &ytwo);
	
	printf("%f", sqrt((xtwo - xone)*(xtwo - xone) + (yone - ytwo)*(yone - ytwo)));
	return 0;
}