#include "bits/stdc++.h"
#include "atcoder/all"
using namespace std;
using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
//const bool debug = false;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
vector<P>to[200005];
int par[200005];
void dfs(int v, int p = -1) {
	for (auto e : to[v]) {
		if (e.first == p) par[v] = e.second;
		else dfs(e.first, v);
	}
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int h, w, n; cin >> h >> w >> n;
	dsu uf(h + w);
	vector<int>x(n), y(n);
	rep(i, n) {
		cin >> x[i] >> y[i]; x[i]--; y[i]--;
		int nx = x[i], ny = h + y[i];
		if (uf.same(nx, ny)) {
			dfs(ny);
			int v = nx;
			vector<int>ans;
			while (v != ny) {
				ans.push_back(par[v]);
				if (v < h)v = y[par[v]] + h;
				else v = x[par[v]];
			}
			ans.push_back(i);
			reverse(all(ans));
			cout << ans.size() << endl;
			rep(i, ans.size()) cout << ans[i] + 1 << " ";
			return 0;
		}
		else {
			uf.merge(nx, ny);
			to[nx].push_back({ ny,i });
			to[ny].push_back({ nx,i });
		}
	}
	cout << -1 << endl;
	return 0;
}