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

typedef long long ll;
typedef string str;
typedef unsigned long long ull;
#define pb push_back
#define mp make_pair

const int MAX_N = 50007;

int sounds[MAX_N];
int sounds1[MAX_N];

int main(){
    ios_base::sync_with_stdio(0), cin.tie(0);
    int n;
    cin >> n;
    for(int i = 0; i < n; ++i) {
        int tmp;
        cin >> tmp;
        sounds[i] = tmp;
        sounds1[i] = tmp;
    }
    int res = 0;
    bool next_higher = true;
    for(int i = 1; i < n; ++i) {
        if(next_higher && sounds[i] <= sounds[i - 1]) {
            res++;
            sounds[i] = sounds[i + 1] + 1;
        }
        else if(!next_higher && sounds[i] >= sounds[i - 1]) {
            res++;
            sounds[i] = sounds[i + 1] - 1;
        }
        if(next_higher) next_higher = false;
        else next_higher = true;
    }
    int res1 = 0;
    next_higher = false;
    for(int i = 1; i < n; ++i) {
        if(next_higher && sounds1[i] <= sounds1[i - 1]) {
            res1++;
            sounds1[i] = sounds1[i + 1] + 1;
        }
        else if(!next_higher && sounds1[i] >= sounds1[i - 1]) {
            res1++;
            sounds1[i] = sounds1[i + 1] - 1;
        }
        if(next_higher) next_higher = false;
        else next_higher = true;
    }
    cout << min(res,res1);
    return 0;
}