View Compilation Error

0_0_26239413_29853.cpp: In function 'll dfs(int, int, int)':
0_0_26239413_29853.cpp:26:47: error: 'ak' was not declared in this scope
  if(o) chkmin(res, dfs(o^1, pos+k, k) - v[pos+ak] + v[pos]);
                                               ^