Intermediate vertices

Given a directed graph and two different vertices $u$ and $v$ , compute how many vertices $x$ different from $u$ and $v$ there are such that there exists some path from $u$ to $v$ passing through $x$ .

Input

The input consists in several cases. Each case begins with $n$ , $u$ , $v$ and $m$ , followed by $m$ different pairs $x$ $y$ , with $x \neq y$ , which indicate an arc that goes from $x$ to $y$ . Assume $2 \leq n \leq 10^4$ , $0 \leq m \leq 10n$ , and that the vertices are numbered between $0$ and $n - 1$ .

Output

For each case, write the number of vertices that can be visited when going from $u$ to $v$ following some path.

Hint

For each case, essentially the expected solution only makes two traversals, each on the right graph.

Problem information

Author: Salvador Roura

Generation: 2026-01-25T22:47:27.299Z