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#include <bits/stdc++.h>
 
using namespace std;
 
int main(void)
{
    ios_base::sync_with_stdio(0);
    int n;
    cin >> n;
    int a[n];
    int b[n];
    
    for(int i = 0; i < n; i++){
        cin >> a[i];
        b[i] = a[i];
    }
    
    if(n == 1){
        cout << 0;
        return 0;
    }
    
    int prev,next,curr;
    int res = 0;
    bool ros = !(a[0] < a[1]);
    for(int i = 1; i < n; i++){
        ros = !ros;
        prev = a[i-1];
        next = i == n-1 ? prev : a[i+1];
        curr = a[i];
        if(prev == curr || (prev < curr && !ros) || (prev > curr && ros)){
            if(ros){
                a[i] = max(prev,next)+1;
            }else{
                a[i] = min(prev,next)-1;
            }
            res++;
        }
    }

    int res2 = 0;
    bool ros2 = b[0] < b[1];
    for(int i = 1; i < n; i++){
        ros2 = !ros2;
        prev = b[i-1];
        next = i == n-1 ? prev : b[i+1];
        curr = b[i];
        if(prev == curr || (prev < curr && !ros2) || (prev > curr && ros2)){
            if(ros2){
                b[i] = max(prev,next)+1;
            }else{
                b[i] = min(prev,next)-1;
            }
            res2++;
        }
    }
    cout << min(res,res2) << endl;
    return 0;
}