#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<vector>
#include<string>
#include<cmath>
#include<queue>
#include<map>
#include<set>
using namespace std;
#define debug printf("-----debug-----\n");
typedef long long ll;
const int MAXN=1005;
vector<int>edges[MAXN];
int cnt[MAXN];
int main()
{
    int n,m;
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        int u,v;
        cin>>u>>v;
        edges[u].push_back(v);
        cnt[v]++;
    }
    queue<int>que;
    for(int i=1;i<=n;i++)
    {
        if(cnt[i]==0)que.push(i);
    }
    int num=0;
    while(!que.empty())
    {
        int u=que.front();que.pop();
        num++;
        for(auto v:edges[u])
        {
            if(--cnt[v]==0)que.push(v);
        }
    }
    cout<<-(num==n)<<endl;
    return 0;
}

### Prompt

Generate a Cpp solution to the following problem:

Given is a directed graph G with N vertices and M edges.
The vertices are numbered 1 to N, and the i-th edge is directed from Vertex A_i to Vertex B_i.
It is guaranteed that the graph contains no self-loops or multiple edges.

Determine whether there exists an induced subgraph (see Notes) of G such that the in-degree and out-degree of every vertex are both 1. If the answer is yes, show one such subgraph.
Here the null graph is not considered as a subgraph.

Constraints

* 1 \leq N \leq 1000
* 0 \leq M \leq 2000
* 1 \leq A_i,B_i \leq N
* A_i \neq B_i
* All pairs (A_i, B_i) are distinct.
* All values in input are integers.

Input

Input is given from Standard Input in the following format:


N M
A_1 B_1
A_2 B_2
:
A_M B_M


Output

If there is no induced subgraph of G that satisfies the condition, print `-1`. Otherwise, print an induced subgraph of G that satisfies the condition, in the following format:


K
v_1
v_2
:
v_K


This represents the induced subgraph of G with K vertices whose vertex set is \\{v_1, v_2, \ldots, v_K\\}. (The order of v_1, v_2, \ldots, v_K does not matter.) If there are multiple subgraphs of G that satisfy the condition, printing any of them is accepted.

Examples

Input

4 5
1 2
2 3
2 4
4 1
4 3


Output

3
1
2
4


Input

4 5
1 2
2 3
2 4
1 4
4 3


Output

-1


Input

6 9
1 2
2 3
3 4
4 5
5 6
5 1
5 2
6 1
6 2


Output

4
2
3
4
5

### Response

```cpp
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<vector>
#include<string>
#include<cmath>
#include<queue>
#include<map>
#include<set>
using namespace std;
#define debug printf("-----debug-----\n");
typedef long long ll;
const int MAXN=1005;
vector<int>edges[MAXN];
int cnt[MAXN];
int main()
{
    int n,m;
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        int u,v;
        cin>>u>>v;
        edges[u].push_back(v);
        cnt[v]++;
    }
    queue<int>que;
    for(int i=1;i<=n;i++)
    {
        if(cnt[i]==0)que.push(i);
    }
    int num=0;
    while(!que.empty())
    {
        int u=que.front();que.pop();
        num++;
        for(auto v:edges[u])
        {
            if(--cnt[v]==0)que.push(v);
        }
    }
    cout<<-(num==n)<<endl;
    return 0;
}

```