#include<cstdio>
#include<cmath>
int main()
{
double xa,xb,ya,yb,len;
scanf("%lf%lf%lf%lf",&xa,&ya,&xb,&yb);
len=sqrt(pow(xa-xb,2) + pow(ya-yb,2));
printf("%.3lf",len);
return 0;
}