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

int main()
{
	double x1, y1, x2, y2, dx, dy;
	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
	dx = x1 - x2;
	dy = y1 - y2;
	printf("%.5f\n", sqrt((dx * dx) + (dy * dy)));
	return 0;
}
