const int LG = 21;
const int FN = 400005;
const long long MOD = 1e9 + 7;
const long long INF = 1e9;
const long long INFLL = 1e18;

#include <iostream>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<ll> vi;

#define forn(i, n) for (int (i) = 0; (i) != (n); (i)++)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define popcount(x) __builtin_popcount(x)
#define popcountll(x) __builtin_popcountll(x)
#define fi first
#define se second
#define re return
#define uniq(x) sort(all(x)); (x).resize(unique(all(x)) - (x).begin())

#ifdef LOCAL
#define dbg(x) cerr << __LINE__ << " " << #x << " " << x << endl
#define ln cerr << __LINE__ << endl
#else
#define dbg(x) void(0)
#define ln void(0)
#endif // LOCAL

int cx[4] = {-1, 0, 1, 0};
int cy[4] = {0, -1, 0, 1};

ll inq(ll x, ll y)
{
    if (!y) re 1 % MOD;
    ll l = inq(x, y / 2);
    if (y % 2) re l * l % MOD * x % MOD;
    re l * l % MOD;
}

ll rev(ll x)
{
    return inq(x, MOD - 2);
}

bool __precomputed_combinatorics = 0;
vector<ll> __fact, __ufact, __rev;

void __precompute_combinatorics()
{
    __precomputed_combinatorics = 1;
    __fact.resize(FN);
    __ufact.resize(FN);
    __rev.resize(FN);
    __rev[1] = 1;
    for (int i = 2; i < FN; i++) __rev[i] = MOD - __rev[MOD % i] * (MOD / i) % MOD;
    __fact[0] = 1, __ufact[0] = 1;
    for (int i = 1; i < FN; i++) __fact[i] = __fact[i - 1] * i % MOD, __ufact[i] = __ufact[i - 1] * __rev[i] % MOD;
}

ll fact(int x)
{
    if (!__precomputed_combinatorics) __precompute_combinatorics();
    return __fact[x];
}

ll cnk(int n, int k)
{
    if (k < 0 || k > n) return 0;
    if (!__precomputed_combinatorics) __precompute_combinatorics();
    return __fact[n] * __ufact[n - k] % MOD * __ufact[k] % MOD;
}

const int N = 6005;

int Root(int x, vi &root)
{
    if (x == root[x]) return x;
    return root[x] = Root(root[x], root);
}

void Merge(int v, int u, vi &root, vi &sz)
{
    v = Root(v, root), u = Root(u, root);
    if (v == u) return;
    if (sz[v] < sz[u])
    {
        sz[u] += sz[v];
        root[v] = u;
    }
    else
    {
        sz[v] += sz[u];
        root[u] = v;
    }
}

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n, K = 0;
    cin >> n;
    vi p(n), root(n), sz(n, 1), mark(n);
    for (int i = 0; i < n; i++) cin >> p[i], p[i]--, root[i] = i;
    for (int i = 0; i < n; i++) if (p[i] != -2) Merge(i, p[i], root, sz);
    for (int i = 0; i < n; i++) if (p[i] == -2) mark[Root(i, root)] = 1, K++;
    vi a, k;
    for (int i = 0; i < n; i++) if (root[i] == i) a.push_back(sz[i]), k.push_back(mark[i]);
    int m = k.size();
    ll ans = inq(n - 1, K) * (n - m + K) % MOD;
    vector<ll> dp(m + 1);
    dp[0] = 1;
    for (int i = 0; i < m; i++)
    {
        if (k[i])
        {
            for (int j = i; j >= 0; j--)
            {
                dp[j + 1] = (dp[j + 1] + dp[j] * a[i]) % MOD;
            }
        }
    }
    for (int len = 2; len <= K; len++)
    {
        long long c = fact(len - 1);
        //cout << c << " " << dp[len] << " " << n - 1 << " " << K - len << endl;
        ans = (ans - c * dp[len] % MOD * inq(n - 1, K - len)) % MOD;
        ans = (ans + MOD) % MOD;
    }
    for (int i = 0; i < m; i++)
    {
        if (k[i]) ans = (ans - (a[i] - 1) * inq(n - 1, K - 1)) % MOD;
        ans = (ans + MOD) % MOD;
    }
    cout << ans;
}

/* Note:
Check constants at the beginning of the code.
    N is set to 4e5 but be careful in problems with large constant factor.
    Setting N in every problem is more effective.
Check corner cases.
    N = 1
No def int long long for now.
Add something here.
*/



Measured memory_bytes: 13100