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#include<cstdio>
#include<vector>

using namespace std;

int changesNum(vector<int>& inversions) {
    int changesToMake = 0;

    for (int i = 0; i < inversions.size(); i++) {
        if (inversions[i] == 2) {
            changesToMake++;
            
            inversions[i - 1]--;
            inversions[i] = 0;
            inversions[i + 1]--;
        }
    }

    for (int i = 0; i < inversions.size(); i++) {
        if (inversions[i] == 1) {
            changesToMake++;

            if (i < inversions.size()) {
                inversions[i + 1]--;
            }
        }
    }

    return changesToMake;
}

int main() {
    int recordLength;
    scanf("%d\n", &recordLength);

    vector<int> lInversions(recordLength, 0);
    vector<int> gInversions(recordLength, 0);

    int last;
    scanf("%d", &last);

    for (int i = 1; i < recordLength; i++) {
        int current;
        scanf("%d", &current);
        if ((current > last && i % 2 == 1) || (current < last && i % 2 == 0) || (current == last)) {
            lInversions[i - 1]++;
            lInversions[i]++;
        }


        if ((current > last && i % 2 == 0) || (current < last && i % 2 == 1) || (current == last)) {
            gInversions[i - 1]++;
            gInversions[i]++;
        }

        last = current;
    }

    int lChanges = changesNum(lInversions);
    int gChanges = changesNum(gInversions);

    if (lChanges <= gChanges) {
        printf("%d\n", lChanges);
    } else {
        printf("%d\n", gChanges);
    }
}