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

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