#include<bits/stdc++.h>
using namespace std;
// #define LOCAL
#define ll long long
#define db double
#define Ciallo true
#define pir pair <ll,ll>
#define fs first
#define sc second
#define mod 1000000007
#define MAXN 5010
inline ll read(){
    ll f = 1, num = 0;
    char c = getchar();
    while(c < '0' || c > '9'){
        if(c=='-') f = -1;c = getchar();
    }
    while(c>='0'&&c<='9') num = num * 10 + c - '0', c = getchar();
    return num * f;
}

int typ[MAXN],p[MAXN];
int n;
ll f[2][MAXN], pre[2][MAXN], suf[2][MAXN];
int main(int argc, char* argv[]){
    freopen("swap.in", "r", stdin);
    freopen("swap.out", "w", stdout);
    #ifdef FS
        freopen(argv[1], "r", stdin);
        freopen(argv[2], "w", stdout);
    #elif defined(LOCAL)
        freopen("w.in", "r", stdin);
        freopen("w.out", "w", stdout);
    #endif

    n = read();
    for(int i = 0;i < n; ++i){
        p[i] = read();
        if(i == p[i]){
            cout << "0\n";
            return 0;
        }

        auto col1 = [&](int x){// 限制 pos_x > pos_x - 1
            if(x <= 0 || x >= n)return;
            if(typ[x] == 2){
                cout << "0\n";
                exit(0);
            }
            typ[x] = 1;
        };

        auto col2 = [&](int x){// 限制 pos_x < pos_x - 1
            if(x <= 0 || x >= n)return;
            if(typ[x] == 1){
                cout << "0\n";
                exit(0);
            }
            typ[x] = 2;
        };

        if(p[i] > i){
            col2(i);
            for(int j = i + 1;j < p[i]; ++j)col1(j);
        }else{
            col1(i);
            for(int j = p[i] + 1;j < i; ++j)col2(j);
        }
    }

    f[0][1] = 1;
    pre[0][1] = suf[0][1] = 1;
    for(int i = 1;i < n - 1; ++i){
        int now = i & 1, lst = now ^ 1;
        memset(f[now], 0, sizeof(f[now]));
        memset(pre[now], 0, sizeof(pre[now]));
        memset(suf[now], 0, sizeof(suf[now]));

        for(int j = 1;j <= i + 1; ++j){
            if(typ[i] == 1)f[now][j] = pre[lst][j - 1];
            if(typ[i] == 2)f[now][j] = suf[lst][j];
        }

        for(int j = 1;j <= i + 1; ++j){
            pre[now][j] = (pre[now][j - 1] + f[now][j]) % mod;
        }
        for(int j = i + 1; j; --j){
            suf[now][j] = (suf[now][j + 1] + f[now][j]) % mod;
        }
    }

    cout << pre[(n - 2) & 1][n - 1] << endl;

    cerr << "Time : " << 1.0 * clock() / CLOCKS_PER_SEC << "s \n";
    return 0;
}