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

int main() {
	double x1, x2, y1, y2;
	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
	printf("%.8lf\n", sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)));
	return 0;
}
