Given $n$ and $t$ lengths $ l_i, 1\leq l_1<l_2<\cdots<l_t\leq n-1$, of directed diagonals within an $n$-gon such that $l_1+\cdots+l_t\neq 0 (mod n)$. Does it exist a directed path within the n-gon using each of the given lengths exactly once (and no vertex twice)? Here the length of a directed diagonal within any kind of an n-gon is the number of sides of the n-gon between the starting and the end vertex counted in a fixed direction. A directed path is a sequence $(d_1,d_2,\ldots,d_t)$ of directed diagonals such that the end vertex of $d_i$ is the starting vertex of $d_{i+1}$ for $i=1,2,…,t-1$. In number theory, it is equivalent to: For any subset $S=\{l_1,\ldots,l_t\}$ of $\{1,2,…,n-1\}$ with $ l_i, 1\leq l_1<l_2<\cdots<l_t\leq n-1$ and $l_1+\cdots+l_t\neq 0 (mod n)$. Is there any a permutation of the elements of the subset $S$ such that no set of consecutive elements in this permutation has sum $=0 (mod n)$. Can anyone give me a counter-example or prove it? I have a proof for case $t\leq 6$. However, I wonder is it true or not in general. Many thanks