Найти число, отличающееся от других чисел (программа Паскаль)

var
  a, b, c, d: integer;
 
begin
  readln(a, b, c, d);
  if (a <> b) and (b = c) and (b = d)
    then writeln(a)
    else if (b <> c) and (a = c) and (a = d)
      then writeln(b)
      else if (c <> d) and (a = b) and (a = d)
        then writeln(c)
        else if (c <> d) and (a = b) and (a = c)
          then writeln(d)
          else writeln('Нет решения');
  readln
end.