Problem B. Topological sorting ≡

Author:StdAlg   Time limit:1 sec
Input file:input.txt   Memory limit:8 Mb
Output file:output.txt  

Statement

You are to write a program that performs a topological sorting of a directed graph. Graph contains N vertices, numbered with integers from 1 to N, and M edges.

In other words, given a partial order on numbers from 1 to N, your program must produce some linear order on these numbers which does not conflict with the given partial order.

Input file format

Input file contains two integers N and M, followed by M pairs of integers. Integers in each pair are different, and represent starting and ending vertex of a graph edge.

These pairs may also be considered comparisons where the first number precedes the second one.

Output file format

Output file must contain a permutation of integers from 1 to N — numbers of vertices sorted in topological order. (That is, representing a linear order.) If ordering is not possible, output file must contain a single number  − 1.

Constraints

0 ≤ N, M ≤ 100000

Sample tests

No. Input file (input.txt) Output file (output.txt)
1 
4 3
1 2
1 3
3 4
 
1 3 2 4
0.157s 0.013s 13