#include <stdio.h>
#include <math.h>
#define rep(i,l,n) for(i=l;i<n;i++)

int main(void){
	double x1,y1,x2,y2,z,ans;
	
	scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
	
	z=pow(x1-x2,2)+pow(y1-y2,2);
	ans=pow(z,0.5);
	
	printf("%f\n",ans);
	return 0;
}