c-gcc, 57 bytes

Result

WA

Duration

563ms

Code [DL]

i;main(x){for(;read(0,&x,1);++i)putchar((x+i-12)%26+65);}

stdin

ELXNCJDCXBJNZQRACZHGIZOVWI

stdout

FNARHPKKGLUZMEGQTRAADVLTVIY

stderr

/tmp/code.c:1:1: warning: data definition has no type or storage class
 i;main(x){for(;read(0,&x,1);++i)putchar((x+i-12)%26+65);}
 ^
/tmp/code.c:1:1: warning: type defaults to 'int' in declaration of 'i' [-Wimplicit-int]
/tmp/code.c:1:3: warning: return type defaults to 'int' [-Wimplicit-int]
 i;main(x){for(;read(0,&x,1);++i)putchar((x+i-12)%26+65);}
   ^~~~
/tmp/code.c: In function 'main':
/tmp/code.c:1:3: warning: type of 'x' defaults to 'int' [-Wimplicit-int]
/tmp/code.c:1:16: warning: implicit declaration of function 'read' [-Wimplicit-function-declaration]
 i;main(x){for(;read(0,&x,1);++i)putchar((x+i-12)%26+65);}
                ^~~~
/tmp/code.c:1:33: warning: implicit declaration of function 'putchar' [-Wimplicit-function-declaration]
 i;main(x){for(;read(0,&x,1);++i)putchar((x+i-12)%26+65);}
                                 ^~~~~~~

strace