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

int main(void) {
	int x1, x2, y1, y2;
	double ans;
	scanf("%d %d %d %d", &x1, &y1, &x2, &y2);
	ans = sqrt(pow((x2 - x1), 2) + pow((y2 - y1), 2));
	printf("%f\n", ans);
	return 0;
}