#pragma GCC target("avx2")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
int main(){
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  vector<long long> dp(N, 0);
  dp[0] = 1;
  vector<unordered_map<int, long long>> next(N);
  for (int i = 0; i < N; i++){
    for (auto &P : next[i]){
      dp[i] += P.second;
      if (i + P.first - 1 < N){
        next[i + P.first - 1][P.first] += P.second;
      }
    }
    dp[i] %= MOD;
    if (i < N - 1 && A[i] != 1){
      dp[i + 1] += dp[i];
      if (dp[i + 1] >= MOD){
        dp[i + 1] -= MOD;
      }
    }
    if (i + A[i] < N){
      next[i + A[i]][A[i] + 1] += dp[i];
    }
  }
  cout << dp[N - 1] << endl;
}