/in/foo.cc: In function 'int main()':
/in/foo.cc:10:26: warning: iteration 3 invokes undefined behavior [-Waggressive-loop-optimizations]
    b[i][j]=a[1][(i-1)*4+j];
            ~~~~~~~~~~~~~~^
/in/foo.cc:8:15: note: within this loop
  for(int i=1;i<=5;i++)
              ~^~~