// O(n^2) のTLE解
#include <bits/stdc++.h>
using namespace std;

int solve() {
  int n;
  cin >> n;
  vector<int> A(n);
  for (int i = 0; i < n; i++) cin >> A[i];

  const int inf = 1e9 + 1e6;
  vector<vector<int>> dp(n+1, vector<int>(n+1, inf));

  dp[0][0] = 0;

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < i + 1; j++) {
      if (dp[i][j] >= inf) continue;
      if (dp[i][j] < A[i]) {
        dp[i+1][j] = min(dp[i+1][j], A[i]);
      }
      dp[i+1][j+1] = min(dp[i+1][j+1], dp[i][j] + 1);
    }
  }

  for (int j = 0; j < n; j++) {
    if (dp[n][j] < inf) {
      return j;
    }
  }
  assert(false);
}

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  cout << solve() << endl;
  return 0;
}