#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const int INF = 1e9;

signed main(){
  int n; cin >> n;
  vector<int> a(n), dp(n, INF);
  for(int i = 0; i < n; i++) scanf("%d", &a[i]);
  int ans = 0;
  for(int i = n - 1; i >= 0; i--){
    if(a[i] == ans) ans--;
  }
  cout << ans << endl;
}