In a certain code language, 'GEFS' is written as 'HFER' and ‘IBIS’ is…

1   2    3    4   5    6   7     8    9  10  11  12  13

 A   B   C    D   E    F   G    H    I    J    K    L   M

 ↓    ↓    ↓    ↓    ↓     ↓   ↓    ↓     ↓   ↓    ↓    ↓    ↓

 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

26  25  24  23  22  21  20  19  18  17  16  15  14

**********************************************************

 THE LOGIC FOLLOWED HERE IS :

FIRST TWO ALPHABETS ARE INCREASED TO 1 POSITIONAL VALUE AND LAST TWO ALPHABETS ARE DECREASED TO 1 POSITIONAL VALUE

G → (+1) → H

E → (+1) → F

F → (-1) → E

S → (-1) → R

AND

I → (+1) → J

B → (+1) → C

I → (-1) → H

S → (-1) → R

SIMILARLY

S → (+1) → T

T → (+1) → U

I → (-1) → H

R → (-1) → Q