Let \( S \) be the set of all pairs \((i,j)\) where \( 1 \leq i < j \leq n \) and \( n \geq 4 \). Any two distinct members of \( S \) are called friends if they have one constituent of the pairs in common, and enemies otherwise.
For example, if \( n = 4 \), then \[ S = \{ (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) \}. \]
Here: - \((1,2)\) and \((1,3)\) are friends, - \((1,2)\) and \((2,3)\) are also friends, - but \((1,4)\) and \((2,3)\) are enemies.