kage1998 Program gt;
label 1;
var a,b,c,d:integer;
begin
read(a,b,c,d);
if(a=b)then begin
if(a=c)then writeln(d) else writeln(c);
goto 1;
end;
if (a=c) then writeln(b) else writeln(a);
1:
end.