crescator2

#include<fstream>
#define DIM 600005
#define mod 1000000007
using namespace std;
int n, i, p, lst, m, sol;
int fact[DIM], inv[DIM], v[DIM];
ifstream fin("crescator.in");
ofstream fout("crescator.out");
int mult(int x, int e){
    if(e == 0){
        return 1;
    }
    else{
        int a = mult(x, e / 2);
        if(e % 2 == 0){
            return a * 1LL * a % mod;
        }
        else{
            return a * 1LL * a % mod * x % mod;
        }
    }
}
int comb(int n, int k){
    return fact[n] * 1LL * inv[k] % mod * inv[n - k] % mod;
}
int cr(int n, int k){
    if(n == 0){
        return 1;
    }
    return comb(n + k - 1, n - 1);
}
int main(){
    fin>> n;
    for(i = 1; i <= n; i++){
        fin>> v[i];
    }
    m = v[n] + n;
    fact[0] = 1;
    for(i = 1; i <= m; i++){
        fact[i] = fact[i - 1] * 1LL * i % mod;
    }
    inv[m] = mult(fact[m], mod - 2);
    for(i = m - 1; i >= 0; i--){
        inv[i] = inv[i + 1] * 1LL * (i + 1) % mod;
    }
    sol = 1;
    lst = 1;
    for(i = 2; i <= n; i++){
        if(v[i] > 0){
            if(v[lst] > v[i]){
                sol = 0;
            }
            if(p == 0 || v[p] < v[lst] || v[p] > v[i]){
                sol = sol * 1LL * cr(v[i] - v[lst] + 1, i - lst - 1) % mod;
            }
            else{
                sol = sol * 1LL * ( cr(v[i] - v[lst] + 1, i - lst - 1) - cr(v[i] - v[p] + 1, i - p - 1) * 1LL * cr(v[p] - v[lst] + 1, p - lst - 1) % mod) % mod;
                if(sol < 0){
                    sol += mod;
                }
            }
            lst = i;
            p = 0;
        }
        else if(v[i] < 0){
            p = i;
            v[p] *= -1;
        }
    }
    fout<< sol;
    return 0;
}