#include <bits/stdc++.h>   using namespace std;   int a[100000];   vector<int> p;   long long fact[100001];   long long inv[100001];   long long finv[100001];   void init(int n) {     int i;     fact[0] = 1;     for (i = 1; i <= n; i++) fact[i] = fact[i - 1] * i % 1000000007;     inv[0] = 1;     inv[1] = 1;     for (i = 2; i <= n; i++)       inv[i] = 1000000007 - (1000000007 / i) * inv[1000000007 % i] % 1000000007;     finv[0] = 1;     for (i = 1; i <= n; i++) finv[i] = finv[i - 1] * inv[i] % 1000000007;   }   long long C(int n, int k) {     return fact[n] * (finv[k] * finv[n - k] % 1000000007) % 1000000007;   }   long long pow(int x, int p) {     long long ans = 1;     for (int i = 0; i < p; i++) ans = ans * x % 1000000007;     return ans;   }   int next(int s, int t, int m) {     if (s < 0 || s >= m) return t;     if (t < 0 || t >= m) return s;     return a[p[s]] < a[p[t]] ? s : t;   }   int pos(int i, int m, int n) {     if (i < 0) return -1;     if (i >= m) return n;     return p[i];   }   long long solve(int cnt, int l, int r, int n) {     int s = cnt - 1, t = cnt + 1;     if (l <= pos(s, p.size(), n) || r >= pos(t, p.size(), n)) return 0;     long long ans = 1;     while (!(s == -1 && t == p.size())) {       int nxt = next(s, t, p.size());       int w = r - l + 1;       if (nxt == t) {         int d = p[nxt] - r - 1;         if (d > a[p[nxt]] - w - 1) return 0;         d = a[p[nxt]] - w - 1 - d;         l -= d;         if (l <= pos(s, p.size(), n)) return 0;         r = p[nxt];         ans = ans * C(a[p[nxt]] - w - 1, d) % 1000000007;         t++;       }       if (nxt == s) {         int d = l - p[nxt] - 1;         if (d > a[p[nxt]] - w - 1) return 0;         d = a[p[nxt]] - w - 1 - d;         r += d;         if (r >= pos(t, p.size(), n)) return 0;         l = p[nxt];         ans = ans * C(a[p[nxt]] - w - 1, d) % 1000000007;         s--;       }     }     ans = ans * C(n - (r - l + 1), l) % 1000000007;     return ans;   }   int main() {     int n, i;     scanf( %d , &n);     init(n);     for (i = 0; i < n; i++) {       scanf( %d , &a[i]);       if (a[i] > 0) p.push_back(i);     }     if (p.size() == 0) {       printf( %I64d n , pow(2, n - 1));       return 0;     }     if (p.size() == 1) {       if (a[p[0]] == 1)         printf( %I64d n , C(n - 1, p[0]));       else {         int t = a[p[0]] - 1;         long long vl = (p[0] >= t ? C(n - t - 1, p[0] - t) : 0);         long long vr = (n - p[0] - 1 >= t ? C(n - t - 1, p[0]) : 0);         printf( %I64d n , pow(2, t - 1) * (vl + vr) % 1000000007);       }       return 0;     }     int cnt = 0;     while (cnt + 1 < p.size() && a[p[cnt]] > a[p[cnt + 1]]) cnt++;     for (i = cnt; i + 1 < p.size(); i++)       if (a[p[i]] > a[p[i + 1]]) {         puts( 0 );         return 0;       }     int l, r;     long long ans;     if (a[p[cnt]] == 1) {       l = p[cnt];       r = p[cnt];       ans = solve(cnt, l, r, n);     } else {       l = p[cnt] - (a[p[cnt]] - 1);       r = p[cnt];       ans = solve(cnt, l, r, n);       l = p[cnt];       r = p[cnt] + (a[p[cnt]] - 1);       ans = (ans + solve(cnt, l, r, n)) % 1000000007;     }     printf( %I64d n , ans * pow(2, r - l - 1) % 1000000007);     return 0;   }