#include <bits/stdc++.h>
#define REP(i, a, n) for(ll i = ((ll) a); i < ((ll) n); i++)
#define INF (1LL << 60)
using namespace std;
typedef long long ll;

const ll SL = 0, SR = 1;
ll N, A[2000], dp[2000][2000][2];

ll dfs(ll l, ll r, ll s) {
  ll &ret = dp[l][r][s];
  if(ret == -1) {
    if(l == 0 && r == N - 1) {
      if(s == SL) ret = A[0];
      if(s == SR) ret = A[N - 1];
    } else {
      ret = INF;
      if(s == SL) {
        if(l > 0) ret = min(ret, dfs(l - 1, r, SL) + 1);
        if(r < N - 1) ret = min(ret, dfs(l, r + 1, SR) + ((r + 1) - l));
        ret = max(ret, A[l]);
      }
      if(s == SR) {
        if(l > 0) ret = min(ret, dfs(l - 1, r, SL) + (r - (l - 1)));
        if(r < N - 1) ret = min(ret, dfs(l, r + 1, SR) + 1);
        ret = max(ret, A[r]);
      }
    }
  }
  return ret;
}

int main(void) {
  cin >> N;
  REP(i, 0, N) cin >> A[i];

  REP(i, 0, N) REP(j, 0, N) dp[i][j][SL] = dp[i][j][SR] = -1;

  ll ans = INF;
  REP(i, 0, N) ans = min(ans, min(dfs(i, i, SL), dfs(i, i, SR)));
  cout << ans << endl;
}