#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N; cin >> N;
    vector<int> A(N);
    rep(i,N) cin >> A[i];

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

    int ans = 0;
    while(!q.empty()) {
        int i = q.front(); q.pop();
        A[i] = min(A[i - 1], A[i + 1]) - 1; ans++;
        if(0 <= i - 2 && A[i - 2] < A[i - 1]) q.push(i - 1);
        if(i + 2 < N && A[i + 1] > A[i + 2]) q.push(i + 1); 
    }

    cout << ans << endl;
}