ACM ProPTIT: B. Xayah.PROPTIT

#include <iostream>
#include <vector>
using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(NULL); cout.tie(NULL);

    int n; cin >> n; vector<int> a(n);
    for (int i=0; i<n; i++) cin >> a[i];

    int MinIndex = 0, MaxIndex = 0;
    int CurrMin = 0, CurrMax = 0;

    for (int i=0; i<n; i++) {
        if (a[i] == 0) {
            CurrMin++; CurrMax++;
            CurrMin = min(CurrMin, 0);
        }
        else {
            CurrMin--; CurrMax--;
            CurrMax = max(CurrMax, 0);
        }
        MinIndex = min(MinIndex, CurrMin);
        MaxIndex = max(MaxIndex, CurrMax);
    }

    cout << (MaxIndex - MinIndex + 1);
    return 0;
}