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

typedef struct{
	double x,y;
}POINT;
double norm(POINT p){
	return p.x * p.x + p.y * p.y;
}
void dif(POINT p, POINT q, POINT *r){
	(*r).x = p.x - q.x;
	(*r).y = p.y - q.y;
}

int main(){
	POINT p1, p2;
	POINT r;
	scanf("%lf %lf %lf %lf", &p1.x, &p1.y, &p2.x, &p2.y);
	dif(p1,p2, &r);
	printf("%lf\n", sqrt(norm(r)));
	return 0;
}