Which letter will replace the question mark (?) in the following series? F4, I27, L16, O125, ?

R216 S36 S64 R36

R36

According to the above alphabet position: F4    ⇒ (6), 22 I27     ⇒ (9), 33 L16   ⇒ (12), 42 O125   ⇒ (15), 53 ?       ⇒ (18), 62 = R36 (add 3 in 1st alphabet position and there is square and cube of consecutive numbers respectively in 2nd alphabet position)