#include <bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
const ll NMAX = 1001;
const ll VMAX = 10001;
const ll INF = (1LL << 55);
const ll MOD = 1000007;
const ll BLOCK = 1000000;
const ll base = 1000000001;
const ll nr_of_bits = 18;
int dp[NMAX][NMAX][3];
int v[NMAX];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, i, j;
cin >> n;
for(i = 1; i <= n; i++){
cin >> v[i];
}
dp[0][0][1] = 1;
int sum = 0;
for(i = 1; i <= n; i++){
int act = i&1;
int last = 1 - act;
for(j = 1; j <= i; j++){
dp[act][j][0] = dp[act][j][1] = dp[act][j][2] = 0;
/// Nu deschid culoare noua
dp[act][j][0] = (dp[last][j][0] * j) % MOD;
dp[i][j][0] += (dp[last][j][1] * min(j, v[i] - 1)) % MOD;
dp[act][j][0] %= MOD;
if(j >= v[i])
dp[act][j][1] = dp[last][j][1];
dp[act][j][2] = (dp[last][j][2] * j) % MOD;
dp[act][j][2] += (dp[last][j][1] * max(0, j - v[i])) % MOD;
dp[act][j][2] %= MOD;
/// Deschid o culoare noua
dp[act][j][0] += dp[last][j - 1][0];
dp[act][j][0] %= MOD;
if(j < v[j]){
dp[act][j][0] += dp[last][j - 1][1];
dp[act][j][0] %= MOD;
}
if(j == v[i]){
dp[act][j][1] += dp[last][j - 1][1];
dp[act][j][1] %= MOD;
}
dp[act][j][2] += dp[last][j - 1][2];
dp[act][j][2] %= MOD;
if(j > v[i]){
dp[act][j][2] += dp[last][j - 1][1] % MOD;
dp[act][j][2] %= MOD;
}
if(i == n){
sum += dp[act][j][0];
sum %= MOD;
}
}
}
cout << sum + 1;
return 0;
}
