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

int main()
{
	double x1, y1, x2, y2;
	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
	printf("%.8f", sqrt(pow((x2 - x1), 2) + pow((y2 - y1), 2)));
	
	return 0;
}