golfscript, 89 bytes

Result

AC

Duration

789ms

Code [DL]

;"#{require'matrix';-Matrix[*$stack[0].to_s.lines.map{|l|l.split.map(&:to_i)<<1}].det/6}"

stdin

56 54 03
23 36 39
20 08 18
69 38 67

stdout

14419

stderr

strace