from collections import deque

n, m = map(int, input().split())
g = [[] for _ in range(n)]
for _ in range(m):
    v, u = map(int, input().split())
    v -= 1
    u -= 1
    g[v].append(u)

def culc_dist(st: int, g: list):
    dist = [1<<60]*n
    q = deque()
    q.append(st)
    dist[st] = 0
    while q:
        v = q.popleft()
        for nv in g[v]:
            if dist[nv] == 1<<60:
                dist[nv] = dist[v] + 1
                q.append(nv)
    return dist

dist1 = culc_dist(0, g)
dist2 = culc_dist(n-2, g)
dist3 = culc_dist(n-1, g)
ans1 = dist1[n-2]+dist2[n-1]+dist3[0]
ans2 = dist1[n-1]+dist3[n-2]+dist2[0]
ans = min(ans1, ans2)
print(ans if ans < 1<<60 else -1)