#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int n; cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    int ans = 0;
    for (int j = 0; j < 3; j++) {
        for (int i = 1; i < n - 1; i++) {
            if (a[i] > max(a[i - 1], a[i + 1])) {
                a[i] = min(a[i - 1], a[ i + 1]) - 1;
                ans++;
            }
        }
        for (int i = n - 2; i > 0; i--) {
            if (a[i] > max(a[i - 1], a[i + 1])) {
                a[i] = min(a[i - 1], a[i + 1]) - 1;
                ans++;
            }
        }
    }
    cout << ans;
}