import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
md = 10**9+7
# md = 998244353

def SCC(to, ot):
    n = len(to)
    fin = [-1]*n
    topo = []
    for u in range(n):
        if fin[u] != -1: continue
        stack = [u]
        while stack:
            u = stack[-1]
            if fin[u] == -1:
                fin[u] = 0
                for v in to[u]:
                    if fin[v] != -1: continue
                    stack.append(v)
            else:
                stack.pop()
                if fin[u] == 0:
                    fin[u] = 1
                    topo.append(u)
    res = []
    while topo:
        u = topo.pop()
        if fin[u] != 1: continue
        fin[u] = 2
        cur = [u]
        i = 0
        while i < len(cur):
            u = cur[i]
            for v in ot[u]:
                if fin[v] == 2: continue
                fin[v] = 2
                cur.append(v)
            i += 1
        res.append(cur)

    return res

n, k = LI()

if k == 0:
    print(1)
    exit()

to = [[] for _ in range(n)]
ot = [[] for _ in range(n)]
for _ in range(k):
    u, v = LI1()
    if u==v:
        print(-1)
        exit()
    to[u].append(v)
    ot[v].append(u)

gg = SCC(to, ot)
if any(len(g) > 1 for g in gg):
    print(-1)
else:
    dist = [1]*n
    ans = 1
    for g in gg[::-1]:
        u = g[0]
        for v in to[u]:
            dist[u] = max(dist[u], dist[v]+1)
        if dist[u] > ans: ans = dist[u]
    print(ans)