#include <bits/stdc++.h>
using namespace std;
int n, a[110], f[110][110][110][2];
bool vis[110][110][110][2];
// 前p个数，第p个数，还剩c0个偶数，c1个奇数，取a[p]=x(x==0偶数，x==1奇数)
int dfs(int p, int c0, int c1, int x) {
    if (a[p] && a[p]%2 != x) return -1;
    if (c0 < 0 || c1 < 0)
        return -1;
    if (!p)
        return 0;
    if (vis[p][c0][c1][x])
        return f[p][c0][c1][x];
    vis[p][c0][c1][x] = true;
    int &u = f[p][c0][c1][x];
    for (int i = 0; i < 2; i++) {
        int tmp = dfs(p-1, c0 - (x%2==0), c1 - (x%2==1), i);
        if (tmp == -1) continue;
        int d = (p > 1) && (i != x);
        if (u == -1 || u > tmp + d)
            u = tmp + d;
    }
    return u;
}
int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        scanf("%d", a+i);
    }
    memset(f, -1, sizeof f);
    int ans = -1;
    for (int i = 0; i < 2; i++) {
        int tmp = dfs(n, n/2, (n+1)/2, i);
        if (tmp != -1)
            if (ans == -1 || ans > tmp)
                ans = tmp;
    }
    printf("%d\n", ans);
    return 0;
}