#include <bits/stdc++.h>
using namespace std;
 
#define INF 1000000000
#define MAX 2505
vector<int> adj[MAX];
int n, m, answer = INF;
 
void dfs(int u, int parent, vector<int>& dist, vector<bool>& visited) {
    visited[u] = true;
    for (int v : adj[u]) {
        if (!visited[v]) {
            dist[v] = dist[u] + 1;
            dfs(v, u, dist, visited);
        }
        else if (v != parent) {
            // Chu trình được tìm thấy: tính độ dài chu trình
            answer = min(answer, dist[u] - dist[v] + 1);
        }
    }
}
 
void solve() {
    scanf("%d %d", &n, &m);
    for (int i = 0; i < m; i++) {
        int u, v; scanf("%d %d", &u, &v);
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
 
    for (int i = 1; i <= n; i++) {
        vector<bool> visited(n + 1, false);
        vector<int> dist(n + 1, 0);
        dfs(i, -1, dist, visited);
    }
 
    if (answer == INF) printf("-1\n");
    else printf("%d\n", answer);
}
 
int main() {
    solve();
}
 

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