#include <bits/stdc++.h>

constexpr long long INF = 1ll << 60;

using namespace std;

int main() {
    int N;
    cin >> N;
    
    vector<long long> A(N, INF);
    
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }

    priority_queue<int> pq;

    for (int i = 1; i < N - 1; i++) {
        if (A[i - 1] < A[i] && A[i] > A[i + 1]) pq.push(i);
    }

    int ans = 0;

    while (!pq.empty()) {
        int i = pq.top();
        pq.pop();

        ++ans;

        A[i] = min(A[i - 1], A[i + 1]) - 1;

        if (i > 1) {
            if (A[i - 2] < A[i - 1] && A[i - 1] > A[i]) pq.push(i - 1);
        }

        if (i < N - 2) {
            if (A[i] < A[i + 1] && A[i + 1] > A[i + 2]) pq.push(i + 1);
        }
    }

    cout << ans << endl;

    return 0;
}