Codeforces celebrates 10 years! We are pleased to announce the crowdfunding-campaign. Congratulate us by the link https://codeforces.com/10years.
×

Virtual contest is a way to take part in past contest, as close as possible to participation on time. It is supported only ICPC mode for virtual contests.
If you've seen these problems, a virtual contest is not for you - solve these problems in the archive.
If you just want to solve some problem from a contest, a virtual contest is not for you - solve this problem in the archive.
Never use someone else's code, read the tutorials or communicate with other person during a virtual contest.

No tag edit access

D. Robot Rapping Results Report

time limit per test

2 secondsmemory limit per test

256 megabytesinput

standard inputoutput

standard outputWhile Farmer John rebuilds his farm in an unfamiliar portion of Bovinia, Bessie is out trying some alternative jobs. In her new gig as a reporter, Bessie needs to know about programming competition results as quickly as possible. When she covers the 2016 Robot Rap Battle Tournament, she notices that all of the robots operate under deterministic algorithms. In particular, robot *i* will beat robot *j* if and only if robot *i* has a higher skill level than robot *j*. And if robot *i* beats robot *j* and robot *j* beats robot *k*, then robot *i* will beat robot *k*. Since rapping is such a subtle art, two robots can never have the same skill level.

Given the results of the rap battles in the order in which they were played, determine the minimum number of first rap battles that needed to take place before Bessie could order all of the robots by skill level.

Input

The first line of the input consists of two integers, the number of robots *n* (2 ≤ *n* ≤ 100 000) and the number of rap battles *m* ().

The next *m* lines describe the results of the rap battles in the order they took place. Each consists of two integers *u*_{i} and *v*_{i} (1 ≤ *u*_{i}, *v*_{i} ≤ *n*, *u*_{i} ≠ *v*_{i}), indicating that robot *u*_{i} beat robot *v*_{i} in the *i*-th rap battle. No two rap battles involve the same pair of robots.

It is guaranteed that at least one ordering of the robots satisfies all *m* relations.

Output

Print the minimum *k* such that the ordering of the robots by skill level is uniquely defined by the first *k* rap battles. If there exists more than one ordering that satisfies all *m* relations, output -1.

Examples

Input

4 5

2 1

1 3

2 3

4 2

4 3

Output

4

Input

3 2

1 2

3 2

Output

-1

Note

In the first sample, the robots from strongest to weakest must be (4, 2, 1, 3), which Bessie can deduce after knowing the results of the first four rap battles.

In the second sample, both (1, 3, 2) and (3, 1, 2) are possible orderings of the robots from strongest to weakest after both rap battles.

Codeforces (c) Copyright 2010-2020 Mike Mirzayanov

The only programming contests Web 2.0 platform

Server time: Feb/18/2020 14:21:48 (g2).

Desktop version, switch to mobile version.

Supported by

User lists

Name |
---|