#include <bits/stdc++.h>

int32_t main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n;
    std::cin >> n;
    std::vector<int> input(n);
    for (int i = 0; i < n; i++) {
        std::cin >> input[i];
    }
    std::vector<int> prf(n + 1, -1), suf(n + 1, -1);
    prf[0] = 0;
    for (int i = 0; i < n; i++) {
        if (input[i] == -1) {
            prf[i + 1] = prf[i];
        } else {
            if (i >= 1 && input[i - 1] == input[i] && prf[i - 1] != -1) {
                prf[i + 1] = prf[i - 1] + 1;
            }
        }
    }
    suf.rbegin()[0] = 0;
    for (int i = 0; i < n; i++) {
        if (input.rbegin()[i] == 1) {
            suf.rbegin()[i + 1] = suf.rbegin()[i];
        } else {
            if (i >= 1 && input.rbegin()[i - 1] == input.rbegin()[i] && suf.rbegin()[i - 1] != -1) {
                suf.rbegin()[i + 1] = suf.rbegin()[i - 1] + 1;
            }
        }
    }
    int ans = -1;
    for (int i = 0; i <= n; i++) {
        if (prf[i] != -1 && suf[i] != -1) {
            if (ans == -1 || ans > prf[i] + suf[i]) {
                ans = prf[i] + suf[i];
            }
        }
    }
    std::cout << ans << "\n";

    return 0;
}