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

int main(void) {
	double x[2], y[2];
	scanf("%lf%lf%lf%lf", &x[0], &x[1], &y[0], &y[1]);
	printf("%lf", sqrt((x[0] - y[0])*(x[0] - y[0]) + (x[1] - y[1]) * (x[1] - y[1])));
	return 0;
}