#include <iostream>


const int MAX_N = 50000;

int N;
int A[50001];

int LiczZmiany(bool up)
{
    int Zmiany = 0;
    int Previous = A[0];

    for (int i = 1; i < N; i++)
    {
        int Current = A[i];
        int Next = A[i+1];
        if (up && Current <= Previous)
        {
            Zmiany++;
            Current = std::max(Previous+1, Next+1);
        }
        else if (!up && Current >= Previous)
        {
            Zmiany++;
            Current = std::min(Previous-1, Next-1);
        }

        up = !up;
        Previous = Current;
    }

    return Zmiany;
}

int main() {

    std::cin >> N;
    for (int i = 0; i < N; i++)
        std::cin >> A[i];
    A[N] = A[N+1];

    int B = LiczZmiany(true);
    int C = LiczZmiany(false);
    int Wynik = std::min(B, C);

    std::cout << Wynik << "\n";

    return 0;
}