#include <bits/stdc++.h>
#define int long long
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
using namespace std;
const int MOD=1e9+7;
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n; cin >> n;
vector<vector<int>> dp(n+1,vector<int>(n+1,0));
vector<int> a(n+1);
vector<int> mx(n+1,0);
for (int i = 0; i < n; i++) cin >> a[i+1];
for (int i = 1; i <= n; i++) mx[i]=max(a[i],mx[i-1]);
for (int i = 1; i <= n; i++)
{
for (int j = 1; j <= n; j++)
{
dp[i][j]=(dp[i-1][j]*j)%MOD;
dp[i][j]=(dp[i][j]+dp[i-1][j-1])%MOD;
}
if(mx[i-1]>0) dp[i][mx[i-1]]=(dp[i][mx[i-1]]+a[i]-1+MOD)%MOD;
}
int sm=1;
for (int i = 1; i <= n; i++) sm=(sm+dp[n][i])%MOD;
cout << sm%MOD << "\n";
return 0;
}
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