#include<bits/stdc++.h>

#define LL long long

#define LD long double

#define fi first

#define se second

#define mk make_pair

#define PLL pair<LL, LL>

#define PLI pair<LL, int>

#define PII pair<int, int>

#define SZ(x) ((int)x.size())

#define ull unsigned long long

using namespace std;

const int N = 1e5 + ;

const int inf = 0x3f3f3f3f;

const LL INF = 0x3f3f3f3f3f3f3f3f;

const int mod = 1e9 + ;

const double eps = 1e-;

const double PI = acos(-);

template<class T, class S> inline void add(T& a, S b) {a += b; if(a >= mod) a -= mod;}

template<class T, class S> inline void sub(T& a, S b) {a -= b; if(a < ) a += mod;}

template<class T, class S> inline bool chkmax(T& a, S b) {return a < b ? a = b, true : false;}

template<class T, class S> inline bool chkmin(T& a, S b) {return a > b ? a = b, true : false;}

int power(int a, int b) {

    int ans = ;

    while(b) {

        if(b & ) ans = 1LL * ans * a % mod;

        a = 1LL * a * a % mod; b >>= ;

    }

    return ans;

}

int inv[N], Finv[N], F[N];

void init() {

    inv[] = F[] = Finv[] = ;

    for(int i = ; i < N; i++) inv[i] = 1LL * (mod - mod / i) * inv[mod % i] % mod;

    for(int i = ; i < N; i++) F[i] = 1LL * F[i - ] * i % mod;

    for(int i = ; i < N; i++) Finv[i] = 1LL * Finv[i - ] * inv[i] % mod;

}

int comb(int n, int m) {

    if(n < m || n < ) return ;

    return 1LL * F[n] * Finv[m] % mod * Finv[n - m] % mod;

}

int n, a[N], prefix[N];

int solve(int L, int R) {

    if(prefix[L - ] == prefix[R]) return power(, R - L);

    int p, q;

    for(p = L; p <= R; p++) if(a[p] != ) break;

    for(q = R; q >= L; q--) if(a[q] != ) break;

    if(p == q && a[p] == ) return comb(R - L, p - L);

    int ans = ;

    if(a[p] >= a[q]) {

        int L2 = p, R2 = L2 + a[p] - ;

        if(R2 >= q && R2 <= R) add(ans, 1LL * solve(L2 + , R2) * comb(R - L - R2 + L2, L2 - L) % mod);

    }

    if(a[q] >= a[p]) {

        int R2 = q, L2 = R2 - a[q] + ;

        if(L2 >= L && L2 <= p) add(ans, 1LL * solve(L2, R2 - ) * comb(R - L - R2 + L2, L2 - L) % mod);

    }

    return ans;

}

int main() {

    init();

    scanf("%d", &n);

    for(int i = ; i <= n; i++) scanf("%d", &a[i]);

    for(int i = ; i <= n; i++) prefix[i] = prefix[i - ] + (a[i] != );

    printf("%d\n", solve(, n));

    return ;

}

/*

*/