program Project2;

{$APPTYPE CONSOLE}

var l,x1,x2,y1,y2 :integer;
    f,t :text;
begin
  assign(f,'storonu.dat');
  reset(f);
  readln(f,x1,y1);
  readln(f,x2,y2);
  close(f);
  assign(t,'perumetr.sol');
  rewrite(t);
  l:=sqrt(sqr(x1-x2)+sqr(y1-y2));
  write(t,l:3);
  close(t);
end.