CN-SMO 2014 Problem 2

Let $n\geq 4$ be a positive integer.Out of $n$ people,each of two individuals play table tennis game(every game has a winner).Find the minimum value of $n$ ,such that for any possible outcome of the game,there always exist an ordered four people group $(a_{1},a_{2},a_{3},a_{4})$ ,such that the person $a_{i}$ wins against $a_{j}$ for any $1\leq i<j\leq 4$