#include <bits/stdc++.h>
#ifdef ULVI
#define db(x) cerr<<"[ "<<#x<<" = "<<(x)<<" ]\n"
#define dbv(v) cerr<<#v<<" = [ ";for(auto &__x : v)cerr<<__x<<' ';cerr<<"]\n"
#define line() cerr<<string(80, '-')<<'\n'
#else
#define db(x)
#define dbv(v)
#define line()
#endif
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define ff first
#define ss second
#define enld endl
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef pair<ll,ll> pll;
const ll sz=2e5+100;
const ll mod=1e9+7;
const ll inf=1e18;
template<class T>
using indexed_set=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
map<pll,ll> dp;
ll f(ll i,ll j){
if(i==0) return 1;
ll &d=dp[{i,j}];
if(!dp.count({i,j})) return d;
return d=(j*f(i-1,j)+f(i-1,j+1))%mod;
}
void solve(){
ll n;
cin>>n;
vector<ll> v(n+5);
for(ll i=1;i<=n;i++) cin>>v[i];
ll ans=1,mx=1;
for(ll i=2;i<=n;i++){
ans=(ans+min(v[i]-1,mx)*f(n-i,mx))%mod;
mx=max(mx,v[i]);
}
cout<<ans<<'\n';
}
int main(){
//freopen("input.txt","r",stdin);
//freopen("output.txt","w",stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);
ll t=1;
//cin>>t;
for(ll _=1;_<=t;_++){
//cout<<"Scenario #"<<_<<":\n";
solve();
}
}
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