#include<iostream.h>
#include<math.h>

struct punct
{ double x,y;
}A,B;

void main()
{ double d;
  cin>>A.x>>A.y;
  cin>>B.x>>B.y;
  d=sqrt((A.x-B.x)*(A.x-B.x) +(A.y-B.y)*(A.y-B.y));
  cout<<d;
}