#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

template<bool directed = false>
struct Graph{
    struct edge{
        int to, id;
        edge(int to, int id) : to(to), id(id) {}
    };

    vector<vector<edge>> es;
    const int n;
    int m;

    vector<int> used;

    Graph(int n) : es(n), n(n), m(0), used(n) {}

    void add_edge(int from, int to){
        es[from].emplace_back(to, m);
        if(!directed) es[to].emplace_back(from, m);
        m++;
    }

    vector<int> ret_path;

    bool detect_path(int now, int t, bool use_id = false){
        used[now] = true;
        if(now == t){
            if(!use_id) ret_path.push_back(now);
            return true;
        }
        for(auto &e : es[now]){
            if(used[e.to]) continue;
            if(detect_path(e.to, t, use_id)){
                ret_path.push_back(use_id? e.id : now);
                return true;
            }
        }
        return false;
    }

    vector<int> find_path(int s, int t, bool use_id = false){ //点素なs-tパスを1つ検出
        ret_path.clear(), fill(begin(used), end(used), 0);
        detect_path(s, t, use_id);
        reverse(begin(ret_path), end(ret_path));
        return ret_path;
    }

    vector<int> ret_cycle;

    int detect_cycle(int now, int pre = -1, bool use_id = false){
        if(used[now]++) return 1;
        for(auto &e: es[now]){
            if(e.id == pre) continue;
            int k = detect_cycle(e.to, e.id, use_id);
            if(k == 2) return 2;
            if(k == 1){
                ret_cycle.push_back(use_id? e.id : now);
                return used[now];
            }
        }
        return 0;
    }

    vector<int> find_cycle(bool use_id = false){ //点素な閉路を1つ検出
        ret_cycle.clear(), fill(begin(used), end(used), 0);
        for(int i = 0; i < n; i++){
            if(used[i]) continue;
            detect_cycle(i, -1, use_id);
            if(!ret_cycle.empty()){
                reverse(begin(ret_cycle), end(ret_cycle));
                return ret_cycle;
            }
        }
        return {};
    }
};

int main(){
    int H, W, N; cin >> H >> W >> N;

    Graph G(H+W);
    vector<int> u(N), v(N);

    rep(i, N){
        int x, y; cin >> x >> y; x--, y--;
        G.add_edge(x, H+y);
        u[i] = x, v[i] = y;
    }

    vector<int> c = G.find_cycle(true);

    if(empty(c)) cout << "-1\n";
    else{
        int n = sz(c);
        rep(i, n){
            int j = (i+1)%n;
            if(v[c[i]] == v[c[j]]){
                cout << n << '\n';
                rep(k, n){
                    cout << c[(i+k)%n]+1 << (k == n-1? '\n' : ' ');
                }
                return 0;
            }
        }
    }
}