from collections import deque
from math import isinf


def bfs(g, s):
    q = deque([s])
    d = [float('inf')] * len(g)
    d[s] = 0
    while q:
        v = q.popleft()
        for u in g[v]:
            if isinf(d[u]):
                d[u] = d[v] + 1
                q.append(u)
    return d


def main():
    n, m = map(int, input().split())
    g = [[] for _ in range(n)]
    for i in range(m):
        a, b = map(int, input().split())
        g[a-1].append(b-1)

    x = bfs(g, 0)
    y = bfs(g, n-2)
    z = bfs(g, n-1)

    ans = min(x[n-2] + y[n-1] + z[0], x[n-1] + z[n-2] + y[0])

    print(-1 if isinf(ans) else ans)


if __name__ == '__main__':
    main()