#include <stdio.h>

typedef struct List {
	struct List *next;
	int v, id;
} list;

int main()
{
	int i, H, W, N, u, w;
	list *adj[200001] = {}, e[200001], *p;
	scanf("%d %d %d", &H, &W, &N);
	for (i = 0; i < N; i++) {
		scanf("%d %d", &u, &w);
		w += H;
		e[i*2].v = u;
		e[i*2].id = i + 1;
		e[i*2].next = adj[w];
		adj[w] = &(e[i*2]);
		e[i*2+1].v = w;
		e[i*2+1].id = i + 1;
		e[i*2+1].next = adj[u];
		adj[u] = &(e[i*2+1]);
	}
	
	int k, ans[2][200001], par[2][200001] = {}, q[200001], head, tail;
	for (i = 1; i <= H + W; i++) {
		if (par[0][i] != 0) continue;
		par[0][i] = i;
		q[0] = i;
		for (head = 0, tail = 1; head < tail; head++) {
			u = q[head];
			for (p = adj[u]; p != NULL; p = p->next) {
				w = p->v;
				if (par[0][w] == 0) {
					par[0][w] = u;
					par[1][w] = p->id;
					q[tail++] = w;
				} else if (w != par[0][u]) break;
			}
			if (p != NULL) break;
		}
		if (head == tail) continue;
		
		ans[0][0] = p->id;
		ans[1][0] = par[1][w];
		for (k = 1, w = par[0][w]; u != w; k++, u = par[0][u], w = par[0][w]) {
			ans[0][k] = par[1][u];
			ans[1][k] = par[1][w];
		}
		break;
	}
	if (i > H + W) printf("-1\n");
	else {
		printf("%d\n", k * 2);
		if (u <= H) {
			for (i = k - 1; i >= 0; i--) printf("%d ", ans[0][i]);
			for (i++; i < k; i++) printf("%d ", ans[1][i]);
			printf("\n");
		} else {
			printf("%d ", ans[1][k-1]);
			for (i = k - 1; i >= 0; i--) printf("%d ", ans[0][i]);
			for (i++; i < k - 1; i++) printf("%d ", ans[1][i]);
			printf("\n");
		}
	}
	fflush(stdout);
	return 0;
}