program vektor_hossza;
var a,b,c,d:real;
function vh(x1,y1,x2,y2:real):real;
begin
     vh:=sqrt(sqr(x1-x2)+sqr(y1-y2))
end;
begin
  readln(a);
  readln(b);
  readln(c);
  readln(d);
  writeln('A hossza: ',vh(a,b,c,d):10:2);
  readln
end.