from collections import deque
from collections import defaultdict

def main():
    N,M = map(int,input().split())
    assert 3<=N<=10**5
    assert 0<=M<=min(10**5,N*(N-1))
    adj_dict = defaultdict(list)
    for _ in [0]*M:
        u,v = map(int,input().split())
        assert 1<=u<=N
        assert 1<=v<=N
        assert u!=v
        assert v not in adj_dict[u]
        adj_dict[u].append(v)
    # 1から各頂点への距離
    dist1 = [-1]*(N+1)
    dist1[1] = 0
    queue = deque([1])
    while queue:
        u = queue.popleft()
        for v in adj_dict[u]:
            if dist1[v]==-1:
                dist1[v] = dist1[u] + 1
                queue.append(v)
    # N-1から各頂点への距離
    distN_1 = [-1]*(N+1)
    distN_1[N-1] = 0
    queue = deque([N-1])
    while queue:
        u = queue.popleft()
        for v in adj_dict[u]:
            if distN_1[v]==-1:
                distN_1[v] = distN_1[u] + 1
                queue.append(v)
    # Nから各頂点への距離
    distN = [-1]*(N+1)
    distN[N] = 0
    queue = deque([N])
    while queue:
        u = queue.popleft()
        for v in adj_dict[u]:
            if distN[v]==-1:
                distN[v] = distN[u] + 1
                queue.append(v)
    ans = []
    if dist1[N-1] != -1 and distN_1[N] != -1 and distN[1] != -1:
        ans.append(dist1[N-1] + distN_1[N] + distN[1])
    if dist1[N] != -1 and distN[N-1] != -1 and distN_1[1] != -1:
        ans.append(dist1[N] + distN[N-1] + distN_1[1])
    if ans == []:
        print(-1)
    else:
        print(min(ans))
    
if __name__ == "__main__":
    main()