N, M = gets.split(" ").map{|s| s.to_i}

r = Array.new(N) {Array.new}

M.times {
	a, b = gets.split(" ").map{|s| s.to_i}
	r[a-1] << (b-1)
	r[b-1] << (a-1)
}

d = Array.new(N, Float::INFINITY)

d[0] = 0

q = [0]

fin = false
while not fin and q.length > 0
	v = q.shift
	r[v].each {|nv|
		if d[nv] > d[v] + 1 then
			d[nv] = d[v] + 1
			q << nv
			if nv == N-1 then
				fin = true
				break
			end
		end		
	}
end

puts fin ? d[N-1] : -1