Realm Succession Sole Claimant

In the realm of Valdris, n heirs, numbered 0 to n-1, have historically pressed claims against one another over the throne. Each recorded claim (a, b) means heir a has an acknowledged stronger claim to the throne than heir b — heir b's ambitions are considered subordinate to heir a's. The royal archive guarantees that no two heirs ever claim superiority over each other in both directions, and no heir is recorded as claiming over themselves. Peace returns to the realm only when a single heir can be recognized as the rightful successor: an heir who is never named as the subordinate side of any recorded claim. Given the full archive of claims, report the rightful successor if the archive identifies exactly one heir meeting this description; otherwise, report that succession remains disputed.

Input format

• The first line contains two integers n and m — the number of heirs and the number of recorded claims.
• Each of the next m lines contains two integers a and b, meaning heir a has an acknowledged stronger claim than heir b.

Output format

• Print a single integer: the index of the rightful successor — the unique heir who never appears as the second value, b, of any claim — or -1 if zero or more than one such heir exists.

Constraints

• 1 <= n <= 100
• 0 <= m <= n * (n - 1) / 2
• 0 <= a, b <= n - 1, a != b
• No claim (a, b) appears more than once, and if (a, b) is recorded then (b, a) is not.

5 4
1 0
1 2
1 3
1 4

1

3 2
1 0
2 0

-1