This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long long
#define mod 1000007
int main(){
    int n;
    cin>>n;
    vector<ll> v(n + 1, 0);
    for (int i = 1; i <= n; i++){
        cin>>v[i];
    }
    vector<vector<ll>> dp(n + 2, vector<ll>(n + 1, 0)), prdp(n + 1, vector<ll>(n + 1, 0));
    for (int i = 0; i <= n; i++){
        dp[n + 1][i] = 1;
    }
    for (int i = n; i >= 1; i--){
        for (int j = 1; j <= i; j++){
            dp[i][j] = dp[i + 1][j] * j;
            dp[i][j] += dp[i + 1][j + 1];
            dp[i][j] %= mod;
        }
    }
    ll ans = 0, la = 0;
    for (int i = 1; i <= n; i++){
        ll su = v[i] - 1;
        for (int j = 1; j <= su; j++){
            ans += dp[i + 1][max(la, (ll)j)];
            ans %= mod;
        }
        ans %= mod;
        la = max(v[i], la);
    }
    cout<<(ans + 1) % mod<<"\n";
}
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