How would I combine these 4 IF statements into one?

=IF(A2="A",IF(ISNUMBER(MATCH("B",$A$2:$A$21,0)),"B",A2),A2)
=IF(B2="D",IF(ISNUMBER(MATCH("E",$A$2:$A$21,0)),"E",B2),B2)
=IF(C2="L",IF(ISNUMBER(MATCH("O",$A$2:$A$21,0)),"O",C2),C2)
=IF(D2="F",IF(ISNUMBER(MATCH("K",$A$2:$A$21,0)),"K",D2),D2)

Any help would be greatly appreciated