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

d = [-1]*n
d[0] = 0
q = [0]
for v in q:
    for c in g[v]:
        if d[c] == -1:
            d[c] = d[v] + 1
            q.append(c)
print(d[-1])