#include<bits/stdc++.h>

using namespace std;

typedef vector<int> vi;

typedef pair<int,int> pi;

#define int long long

#define F first

#define S second

#define PB push_back

#define MP make_pair

#define REP(i, a, b) for(int i =a;i < b; i++)

const int MX = 2e5 + 3;

vi adj[MX];

int mark[MX],dis[MX], ans;

void DFS(int u, int par, int k);

int32_t main()

{

ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);

int n, m;

int u, v;

cin>>n>>m;

ans = m + 1;

REP(i, 0, m)

{

cin>>u>>v;

adj[v].PB(u);

adj[u].PB(v);

}

REP(i, 1, n+1)

{

dis[i]=0;

DFS(i, -1, mark[i]);

}

if(ans == m+1) cout<<-1;

else cout<<ans;

return 0;

}

void DFS(int u, int par, int k)

{

mark[u] ++;

if(u == par)

{

ans = min(ans, (int)1);

}

for(auto v: adj[u])

{

if(mark[v] == k)

{

dis[v] = dis[u]+1;

DFS(v, u, k);

}

else if(v != par)

{

ans = min(ans, dis[u]+dis[v]+1);

}

}

}