from collections import deque
n, m = map(int, input().split())
g = [[] for _ in range(n)]
for i in range(m):
    u, v = map(lambda x: x-1, map(int, input().split()))
    g[u].append(v)
    
q = deque([0])
dist = [-1]*n
dist[0] = 0

while q:
    v = q.popleft()
    for v2 in g[v]:
        if dist[v2]!=-1: continue
        dist[v2] = dist[v] + 1
        q.append(v2)
        
if dist[n-2]==-1 and dist[n-1]==-1: exit(print(-1))

#  町nから
q = deque([n-1])
dist2 = [-1]*n
dist2[n-1] = 0

while q:
    v = q.popleft()
    for v2 in g[v]:
        if dist2[v2]!=-1: continue
        dist2[v2] = dist2[v] + 1
        q.append(v2)
        
#  町n-1から
q = deque([n-2])
dist3 = [-1]*n
dist3[n-2] = 0

while q:
    v = q.popleft()
    for v2 in g[v]:
        if dist3[v2]!=-1: continue
        dist3[v2] = dist3[v] + 1
        q.append(v2)
        
ans = 10**18
if dist[n-2]!=-1 and dist[n-1]==-1:
    if dist3[n-1]!=-1 and dist2[0]!=-1:
        ans = min(ans, dist[n-2] + dist3[n-1] + dist2[0])
    if dist3[n-1]!=-1 and dist2[n-2]!=-1 and dist3[0]!=-1:
        ans = min(ans, dist[n-2]+dist3[n-1]+dist2[n-2]+dist3[0])
elif dist[n-2]==-1 and dist[n-1]!=-1:
    if dist2[n-2]!=-1 and dist3[0]!=-1:
        ans = min(ans, dist[n-1]+dist2[n-2]+dist3[0])
    if dist2[n-2]!=-1 and dist3[n-1]!=-1 and dist2[0]!=-1:
        ans = min(ans, dist[n-1]+dist2[n-2]+dist3[n-1]+dist2[0])
else:
    if dist3[n-1]!=-1 and dist2[0]!=-1:
        ans = min(ans, dist[n-2] + dist3[n-1] + dist2[0])
    if dist3[n-1]!=-1 and dist2[n-2]!=-1 and dist3[0]!=-1:
        ans = min(ans, dist[n-2]+dist3[n-1]+dist2[n-2]+dist3[0])
    if dist2[n-2]!=-1 and dist3[0]!=-1:
        ans = min(ans, dist[n-1]+dist2[n-2]+dist3[0])
    if dist2[n-2]!=-1 and dist3[n-1]!=-1 and dist2[0]!=-1:
        ans = min(ans, dist[n-1]+dist2[n-2]+dist3[n-1]+dist2[0])

if ans==10**18: print(-1)
else: print(ans)