#include<stdio.h>
#include<math.h>
int main()
{
	double x1, x2, y1, y2, x, y, kyori;

	scanf(" %lf %lf %lf %lf", &x1, &y1, &x2, &y2);

	x = x2 - x1;
	y = y2 - y1;

	x = pow(x,2);
	y = pow(y,2);

	kyori = x + y;

	kyori = sqrt(kyori);

	printf("%f", kyori);

	return 0;
}