#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) begin(a),end(a)
using ll = long long;
using ar2 = array<ll,2>;
using vi = vector<int>;
const int mxN = (int)1e4+10;
const ll INF = (int)1e9;
const ll MOD = (ll)1e6+7;
ll n, mx, xd, ans=1;
ll a[mxN], dp[2][mxN][2];
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n;
for(int i = 0; i < n; i++) cin >> a[i], a[i]--;
for(int i = 0; i < n; i++) dp[n&1][i][0]=dp[n&1][i][1]=1;
for(int i = n-1,ii=(n-1)&1,pi=n&1; i>=0; i--,ii^=1,pi^=1){
for(int j = 0; j <= i; j++){
for(int sm = 0; sm <= 1; sm++){
dp[ii][j][sm] = 0;
int lim = sm?n-1:a[i];
if(j+1<lim){
dp[ii][j][sm]+=dp[pi][j][1]*(j+1);
dp[ii][j][sm]+=dp[pi][j+1][1];
}
else{
if(lim<=j){
dp[ii][j][sm]+=dp[pi][j][1]*lim+dp[pi][j][sm];
}
else{
dp[ii][j][sm]+=dp[pi][j][1]*(j+1)+dp[pi][j+1][sm];
}
}
dp[ii][j][sm]%=MOD;
}
}
}
cout << dp[0][0][0] << "\n";
}
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