#include <stdio.h>

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

void chmax(int* a, int b)
{
	if (*a < b) *a = b;
}

int main()
{
	int i, N, M, u, w, deg[100001] = {};
	list *adj[100001] = {}, e[100001], *p;
	scanf("%d %d", &N, &M);
	for (i = 0; i < M; i++) {
		scanf("%d %d", &u, &w);
		e[i].v = w;
		e[i].next = adj[u];
		adj[u] = &(e[i]);
		deg[w]++;
	}
	
	int q[100001], dp[100001] = {}, head, tail = 0;
	for (u = 1; u <= N; u++) if (deg[u] == 0) q[tail++] = u;
	for (head = 0; head < tail; head++) {
		u = q[head];
		chmax(&(dp[0]), dp[u]);
		for (p = adj[u]; p != NULL; p = p->next) {
			w = p->v;
			chmax(&(dp[w]), dp[u] + 1);
			deg[w]--;
			if (deg[w] == 0) q[tail++] = w;
		}
	}
	if (tail != N) printf("-1\n");
	else printf("%d\n", dp[0] + 1);
	fflush(stdout);
	return 0;
}