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

int main(void)
{
	double a,b,c,d,e;
        double A,B,C,D,E;

	scanf("%lf %lf %lf %lf",&a,&b,&c,&d);
	A = c - a;
	B = d - b;
	C = pow(A,2);
	D = pow(B,2);
	e = C + D;
        E = sqrt(e);
	printf("%lf\n",E);
	return 0;
}