#include <bits/stdc++.h>  
using namespace std;  
#define maxn 411  
#define INF 11111  
  
int a1[maxn][maxn], a2[maxn][maxn];  
int n, m;  
int d[maxn];  
bool vis[maxn];  
  
int dij (int a[maxn][maxn]) {  
    for (int i = 1; i <= n; i++) {  
        d[i] = INF;  
        vis[i] = 0;  
    }  
    d[1] = 0;  
    for (int j = 1; j <= n; j++) {  
        int k = -1;  
        int Min = INF;  
        for (int i = 1; i <= n; i++) {  
            if (!vis[i] && d[i] < Min) {  
                Min = d[i];  
                k = i;  
            }  
        }  
        if (k == -1)  
            break;  
        vis[k] = 1;  
        for (int i = 1; i <= n; i++) {  
            if (!vis[i])  
                d[i] = min (d[i], d[k]+a[k][i]);  
        }  
    }  
    return d[n];  
}  
  
int main () {  
    scanf ("%d%d", &n, &m);  
    for (int i = 1; i <= n; i++) {  
        for (int j = 1; j <= n; j++) {  
            a1[i][j] = a2[i][j] = INF;  
        }  
    }  
    int u, v;  
    for (int i = 1; i <= m; i++) {  
        scanf ("%d%d", &u, &v);  
        a1[u][v] = a1[v][u] = 1;  
    }  
    for (int i = 1; i <= n; i++) {  
        for (int j = 1; j <= n; j++) {  
            if (i == j)  
                continue;  
            if (a1[i][j] == INF)  
                a2[i][j] = 1;  
        }  
    }  
    int ans1 = dij (a1);  
    int ans2 = dij (a2);   
    if (ans1 >= INF || ans2 >= INF)  
        cout << "-1" << endl;  
    else cout << max (ans1, ans2) << endl;  
    return 0;  
}