#include <iostream>
#include <sstream>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <cstring>
#include <bitset>
#include <functional>
#include <random>
#define REP(_i,_a,_n) for(int _i=_a;_i<=_n;++_i)
#define PER(_i,_a,_n) for(int _i=_n;_i>=_a;--_i)
#define hr putchar(10)
#define pb push_back
#define lc (o<<1)
#define rc (lc|1)
#define mid ((l+r)>>1)
#define ls lc,l,mid
#define rs rc,mid+1,r
#define x first
#define y second
#define io std::ios::sync_with_stdio(false)
#define endl '\n'
#define DB(_a) ({REP(_i,1,n) cout<<_a[_i]<<',';hr;})
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const int P = 1e9+7, INF = 0x3f3f3f3f;
ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}
ll qpow(ll a,ll n) {ll r=1%P;for (a%=P;n;a=a*a%P,n>>=1)if(n&1)r=r*a%P;return r;}
ll inv(ll x){return x<=1?1:inv(P%x)*(P-P/x)%P;}
inline int rd() {int x=0;char p=getchar();while(p<'0'||p>'9')p=getchar();while(p>='0'&&p<='9')x=x*10+p-'0',p=getchar();return x;}
//head



const int N = 5e3+10;
int n,m,cnt,fa[N],sz[N],p[N],f[N][N],fac[N],pw[N],c[N];
int Find(int x) {return fa[x]?fa[x]=Find(fa[x]):x;}
void add(int x, int y) {
	x=Find(x),y=Find(y);
	if (x!=y) fa[x]=y,sz[y]+=sz[x];
	else ++cnt;
}

int main() {
	scanf("%d", &n);
	REP(i,1,n) sz[i] = 1;
	REP(i,1,n) { 
		scanf("%d", p+i);
		if (p[i]>0) add(i,p[i]);
	}
	fac[0] = pw[0] = 1;
	REP(i,1,n) fac[i]=(ll)fac[i-1]*i%P,pw[i]=(ll)pw[i-1]*(n-1)%P;
	REP(i,1,n) if (p[i]==-1) { 
		++m;
		c[m] = sz[Find(i)];
		f[1][m] = (f[1][m-1]+c[m])%P;
	}
	int ans = (ll)(f[1][m]-m)*pw[m-1]%P;
	REP(i,2,m) {
		REP(j,1,m) f[i][j]=(f[i][j-1]+(ll)c[j]*f[i-1][j-1])%P;
		ans = (ans+(ll)f[i][m]*pw[m-i]%P*fac[i-1])%P;
	}
	ans = ((ll)(n-cnt)*pw[m]-ans)%P;
	if (ans<0) ans += P;
	printf("%d\n", ans);
}
