Secret encryption 1 can you guess the key?

1

if

a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26

Decode

19 16 13 12 16 26 13 28 21

If additive encryption scheme is not told to you n+x

[Program finished]