// #pragma GCC target ("avx,avx2,fma")
// #pragma GCC optimize ("Ofast,inline") // O1 - O2 - O3 - Os - Ofast
// #pragma GCC optimize ("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (ll i = (a); i < (b); ++i)
#define per(i, a, b) for (ll i = (b) - 1; i >= (a); --i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (ll)x.size()
#define pb push_back
#define debug(x) cout<<#x<<" = "<<x<<endl
#define umap unordered_map
#define uset unordered_set
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<vi> vvi;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<ll,pll> ppll;
typedef vector<ll> vll;
typedef vector<pll> vpll;
typedef vector<vll> vvll;
const ll INF = 1'000'007;
int main() {
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit);
ll n;
cin>>n;
vll order(n);
rep(i,0,n) cin>>order[i];
vector<vll> dp(n+1,vll(2,0)),dp2(n+1,vll(2,0));
dp[0][1]=1;
rep(i,0,n) {
rep(j,0,i+1) {
dp2[j][0]=(dp2[j][0]+dp[j][0]*j)%INF;
dp2[j+1][0]=(dp2[j+1][0]+dp[j][0])%INF;
if(j+1==order[i]) dp2[j+1][1]=(dp2[j+1][1]+dp[j][1])%INF;
else dp2[j][1]=(dp2[j][1]+dp[j][1])%INF;
dp2[j][0]=(dp2[j][0]+dp[j][1]*(order[i]-1))%INF;
dp[j][0]=0;
dp[j][1]=0;
}
dp.swap(dp2);
}
ll ans=0;
rep(i,0,n+1) rep(j,0,2) ans=(ans+dp[i][j])%INF;
cout<<ans<<endl;
return 0;
}
