import std, core.bitop;
string[] _R;
void readM(T)(ref T x) { while (_R.empty) { _R = readln.chomp.split; } x = _R.front.to!T; _R.popFront; }
bool chmin(T)(ref T A, T B) { if (A > B) { A = B; return true; } else { return false; } }
bool chmax(T)(ref T A, T B) { if (A < B) { A = B; return true; } else { return false; } }
int lowerBound(T)(T[] A, T x) { int lo = -1, hi = cast(int)(A.length); while (hi - lo > 1) { int mid = lo + hi >> 1; (A[mid] < x ? lo : hi) = mid; } return hi; }
int upperBound(T)(T[] A, T x) { int lo = -1, hi = cast(int)(A.length); while (hi - lo > 1) { int mid = lo + hi >> 1; (A[mid] > x ? hi : lo) = mid; } return hi; }

enum INF = 10 ^^ 8;

int N, M;
int[][] G;

int[] BFS(int s) {
	auto dist = new int[N];
	dist[] = INF;
	dist[s] = 0;
	int[] que;
	que ~= s;
	while (!que.empty) {
		int u = que.front;
		que.popFront;
		if (!u) {
			continue;
		}
		foreach (v; G[u]) {
			if (chmin(dist[v], dist[u] + 1)) {
				que ~= v;
			}
		}
	}
	return dist;
}

void main() {
	N.readM;
	M.readM;
	G = new int[][](N, 0);
	foreach (i; 0 .. M) {
		int u, v;
		u.readM;
		v.readM;
		--u, --v;
		G[u] ~= v;
		G[v] ~= u;
	}
	auto A = BFS(N - 2);
	auto B = BFS(N - 1);
	int ans = INF;
	chmin(ans, A[0] + A[N - 1] + B[0]);
	chmin(ans, B[0] + B[N - 2] + A[0]);
	if (ans == INF) {
		ans = -1;
	}
	writeln(ans);
}