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

int main(){
	double x1,x2,y1,y2,kyori;
	char input[128],*p;
	
	fgets(input,sizeof input,stdin);
	p=strtok(input," ");
	x1=strtod(p,NULL);
	p=strtok(NULL," ");
	y1=strtod(p,NULL);
	p=strtok(NULL," ");
	x2=strtod(p,NULL);
	p=strtok(NULL," ");
	y2=strtod(p,NULL);
	
	printf("%.6f\n",hypot(fabs(x1-x2),fabs(y1-y2)));
	return 0;
}