#include <bits/stdc++.h>
#include <unistd.h>
#include <sys/time.h>
#include <stdlib.h>
 
using namespace std;
 
typedef long long ll;
typedef pair<ll,ll> P;
typedef pair<P, ll> T;
typedef pair<long double , ll> Ps;
 
const ll INF = 2e18;
const ll fact_table = 3200008;
long double Pi = 3.1415926535897932384626;

priority_queue <ll> pql;
priority_queue <P> pqp;
priority_queue <P> bag;
//big priority queue
priority_queue <ll,vector<ll>,greater<ll> > pqls;
priority_queue <P,vector<P>,greater<P> > pqps;
//small priority queue
//top pop
 
ll dx[8]={1,0,-1,0,1,1,-1,-1};
ll dy[8]={0,1,0,-1,1,-1,-1,1};
char dir[] = "DRUL";
//↓,→,↑,←

#define endl "\n"
#ifdef ENJAPMA
    #undef endl
#endif
#define p(x) cout<<x<<endl;
#define el cout<<endl;
#define pe(x) cout<<x<<" ";
#define ps(x) cout<<fixed<<setprecision(25)<<x<<endl;
#define pu(x) cout<<(x);
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define CLEAR(a) a = decltype(a)(); 
#define pc(x) cout << x << ",";
#define rep(i, n) for (ll i = 0; i < (n); i ++)

// const ll mod = 998244353ll;
const ll mod = 1000000007ll;

ll mypow(ll a, ll b, ll mod){ll x=1;while(b){while(!(b&1)){(a*=a)%=mod;b>>=1;}(x*=a)%=mod;b--;}return x;}
void YES(bool cond){ if(cond){ p("YES");}else{ p("NO");} return;}
void Yes(bool cond){ if(cond){ p("Yes");}else{ p("No");} return;}


ll fact[fact_table + 5],rfact[fact_table + 5]; 
 
void c3_init(){
    fact[0] = rfact[0] = 1;
    for(ll i=1; i<=fact_table; i++){
        fact[i] = (fact[i-1]*i) % mod;
    }
    rfact[fact_table] = mypow(fact[fact_table],mod - 2, mod);
    for(ll i=fact_table; i>=1; i--){
       rfact[i-1] = rfact[i] * i;
       rfact[i-1] %= mod;
    }
    return;}
ll c3(ll n,ll r){
    return (((fact[n] * rfact[r]) % mod ) * rfact[n-r]) % mod;}


bool multicase = false;

ll n,m,num,a,b,c,d,e,h,q;
ll w, k, ans;

ll x[500005], y[500005], z[500005];
ll dp[5005][5005];
P table[500005];
string s;

ll parent[500005], sizes[500005];
bool one[500005];

void init(ll n){
    for(ll i=0;i<=n;i++){
        parent[i] = i;
        sizes[i] = 1;
    }
}
ll find(ll x){
    if(parent[x] == x){
        return x;
    }else{
        return parent[x] = find(parent[x]);
    }
}

void unite(ll x,ll y){
    x = find(x);
    y = find(y);
    if(x == y)return;
    if(sizes[x] < sizes[y])swap(x, y);

    parent[y] = x;
    sizes[x] += sizes[y];
}

bool same(ll x,ll y){
    return find(x) == find(y);
}

ll size(ll x){
    return sizes[find(x)];
}

void tellme(ll num){
    for(ll i=1;i<=400;i++){
        for(ll j=1;j<=400;j++){
            ll res = i * mypow(j, mod - 2, mod);
            res %= mod;
            if(res == num){
                pe(j);pe("分の");p(i);
                return;
            }
        }
    }
    p("I dont know");
    return;
}

void solve(){
    cin >> n;
    c3_init();
    init(n + 100);
    for(ll i=1;i<=n;i++){
        cin >> x[i];
        y[i] = INF;
        if(x[i] == -1)continue;
        unite(i, x[i]);
    }
    for(ll i=1;i<=n;i++){
        y[find(i)] = min(y[find(i)], i);
        if(x[i] == -1)one[find(i)] = true;
    }
    vector<ll> v, u;
    for(ll i=1;i<=n;i++){
        if(y[i] == INF)continue;
        if(one[i]) v.pb(sizes[i]);
        if(!one[i]) u.pb(sizes[i]);
    }

    dp[0][0] = 1;
    for(ll i=0;i<v.size();i++){
        for(ll j=0;j<=v.size();j++){
            dp[i+1][j+1] += dp[i][j] * v[i];
            dp[i+1][j] += dp[i][j];
            dp[i+1][j+1] %= mod;
            dp[i+1][j] %= mod;
        }
    }
    ll ans = 0;
    for(ll i=2;i<=v.size();i++){
        ll res = fact[i-1] * dp[v.size()][i];
        res %= mod;
        res *= mypow(n - 1, v.size() - i, mod);
        res %= mod;
        ans += res;
        ans %= mod;
    }
    if(v.size() >= 1){
        ll pro = 0;
        for(ll i=0;i<v.size();i++){
            pro += (v[i] - 1);
            pro %= mod;
        }
        pro *= mypow(n - 1, v.size() - 1, mod);
        pro %= mod;
        ans += pro;
        ans %= mod;
    }
    ans += (ll)u.size() * mypow(n - 1, v.size(), mod);
    ans %= mod;
    ll res = n * mypow(n - 1, v.size(), mod);
    res %= mod;
    res = res + (mod - ans);
    res %= mod;
    p(res);
    return;
}


int main(){
    // init();
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll q, testcase = 1;
    if(multicase){
        cin >> q;
    }else{
        q = 1; 
    }
    while(q--){
        // pu("Case ");pu("#");pu(testcase);pu(": ");
        solve();
        testcase++;
    }
    // solve();
    return 0;
}
















Measured memory_bytes: 249416