#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

const ll MOD = 1000000007;

struct Node {
  int idx;
  int v;

  Node(int idx = -1, int v = -1) {
    this->idx = idx;
    this->v = v;
  }

  bool operator>(const Node &n) const {
    return v < n.v;
  }
};

int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  priority_queue <Node, vector<Node>, greater<Node>> pque;
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
    if (1 <= i && i <= N - 2) {
      pque.push(Node(i, A[i]));
    }
  }

  int ans = 0;

  while (not pque.empty()) {
    Node node = pque.top();
    pque.pop();

    int left = node.idx - 1;
    int right = node.idx + 1;
    // fprintf(stderr, "(%d, %d, %d)\n", A[left], A[node.idx], A[right]);

    if (A[left] < A[node.idx] && A[node.idx] > A[right]) {
      int nv = min(A[left], A[right]) - 1;
      A[node.idx] = nv;
      ++ans;
      pque.push(Node(node.idx, nv));
    }
  }

  cout << ans << endl;

  return 0;
}