beehives2.cpp

// Michael Pfeifer
// Version 04, 2018/02/17

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef unsigned long long ull;

const ld  pi   = 4.0*atanl(1.0);
const int iinf = 1e9 + 10;
const ll  inf  = 1e18 + 10;
const int mod  = 1000000007;
const ld  prec = .000001;

#define enld endl
#define endl '\n'
#define pb push_back
#define debug(x) cout<<#x<<" -> "<<x<<'\n'
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define uni(x) (x).erase(unique(all(x)), (x).end())
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define umap unordered_map
#define uset unordered_set

template<class TIn>
using indexed_set = tree<
        TIn, null_type, less<TIn>,
        rb_tree_tag, tree_order_statistics_node_update>;

void fast() {
    ios::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
}

void file() {
    auto a = freopen("a.in",  "r", stdin);
    auto b = freopen("a.out", "w", stdout);
    if(!a || !b) cout << "uh oh" << endl;
}





int n, m;

int bfs(vector<uset<int>>& adj, int start) {
    vector<int> vis(n, iinf);

    queue<pair<int,int>> q;
    q.push({start, -1});
    vis[start] = 1;

    int girth = iinf;
    while(!q.empty()) {
        int curr = q.front().first;
        int skip = q.front().second;
        q.pop();

        for(auto next : adj[curr]) {
            if(next == skip) {
                continue;
            }

            if(vis[next] != iinf) {
                girth = min(girth, vis[curr] + vis[next] - 1);
            }
            else {
                if(vis[next] > vis[curr]+1) {
                    vis[next] = vis[curr] + 1;
                    q.push({next,curr});
                }
            }
        }
    }

    return girth;
}

int main() {
    //file();
    fast();

    cin >> n >> m;

    vector<uset<int>> adj(n);
    for(int i = 0; i < m; i++) {
        int n1, n2;
        cin >> n1 >> n2;
        adj[n1].insert(n2);
        adj[n2].insert(n1);
    }

    int girth = iinf;
    for(int i = 0; i < n; i++) {
        girth = min(girth, bfs(adj, i));
    }

    if(girth == iinf) {
        cout << "impossible" << endl;
    }
    else {
        cout << girth << endl;
    }

    return 0;
}