#include<bits/stdc++.h>

using namespace std;

typedef long long int64;

inline void chmin(int64 &a, int64 b) { a = min(a, b); }

inline void chmax(int64 &a, int64 b) { a = max(a, b); }

const int64 INF = 1LL << 55;
int N, A[2003];
int64 dp[2003][2003][2];

int main()
{
  cin >> N;
  assert(1 <= N && N <= 2000);
  for(int i = 1; i <= N; i++) {
    cin >> A[i];
    assert(0 <= A[i] && A[i] <= 1000000000);
  }

  fill_n(**dp, 2003 * 2003 * 2, INF);

  dp[2][N][0] = A[1];
  dp[1][N - 1][1] = A[N];

  for(int i = N - 2; i >= 0; i--) {
    for(int j = 1; j + i <= N; j++) {
      const int L = j, R = j + i;
      chmin(dp[L + 1][R][0], dp[L][R][0] + 1);
      chmin(dp[L + 1][R][0], dp[L][R][1] + i + 1);
      chmax(dp[L + 1][R][0], A[L]);
      chmin(dp[L][R - 1][1], dp[L][R][0] + i + 1);
      chmin(dp[L][R - 1][1], dp[L][R][1] + 1);
      chmax(dp[L][R - 1][1], A[R]);
    }
  }

  int64 ret = INF;
  for(int i = 1; i <= N + 1; i++) {
    chmin(ret, dp[i][i - 1][0]);
    chmin(ret, dp[i][i - 1][1]);
  }
  cout << ret << endl;
}