c-gcc, 41 bytes

Result

WA

Code [DL]

main(a,b){b=21;a=sqrt(b);printf("%d",a);}

stdin

01111000
01101000
10111110
00001110
00000001
00010100
00000011
11010000
10101001
00000001
11111101
00000110
10101111
00110111
10011001
10001110
10000011
00001111
11100111
01101001
10011111
01000100
01011011
01101101
01100101
01011001
00100100
00011100
00101101
10001000
01111001
00010101
10000111
10110000
10011100
10110001
00001010
01001110
00111100
01000010
10011001
11111000
11011101
11010010
11111110
00000000
11010110
10101011
01010000
11110110

stdout

4

stderr

/tmp/code.c:1:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 main(a,b){b=21;a=sqrt(b);printf("%d",a);}
 ^
/tmp/code.c: In function ‘main’:
/tmp/code.c:1:1: warning: type of ‘a’ defaults to ‘int’ [-Wimplicit-int]
/tmp/code.c:1:1: warning: type of ‘b’ defaults to ‘int’ [-Wimplicit-int]
/tmp/code.c:1:18: warning: implicit declaration of function ‘sqrt’ [-Wimplicit-function-declaration]
 main(a,b){b=21;a=sqrt(b);printf("%d",a);}
                  ^
/tmp/code.c:1:18: warning: incompatible implicit declaration of built-in function ‘sqrt’
/tmp/code.c:1:18: note: include ‘<math.h>’ or provide a declaration of ‘sqrt’
/tmp/code.c:1:26: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
 main(a,b){b=21;a=sqrt(b);printf("%d",a);}
                          ^
/tmp/code.c:1:26: warning: incompatible implicit declaration of built-in function ‘printf’
/tmp/code.c:1:26: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’

strace