/in/foo.cc: In function 'int main()':
/in/foo.cc:26:12: error: missing template arguments before 'dp'
     vector dp(2 * n + 1,vector<int>(2 * n + 1));
            ^~
/in/foo.cc:31:17: error: 'dp' was not declared in this scope
                 dp[i][j] = max(dp[i][j],dp[i][k] + dp[k + 1][j] + head[i] * head[k + 1] * tail[j]);
                 ^~
/in/foo.cc:37:23: error: 'dp' was not declared in this scope
         ans = max(ans,dp[i][i + n - 1]);
                       ^~