include

include

include

using namespace std;

string fairRations(vector B) { int loaves = 0; int n = B.size();

for (int i = 0; i < n - 1; ++i) {
    if (B[i] % 2 != 0) {
        B[i]++;
        B[i + 1]++;
        loaves += 2;
    }
}

if (B[n - 1] % 2 != 0) {
    return "NO";
} else {
    return to_string(loaves);
}

}

int main() { ios::sync_with_stdio(false); cin.tie(NULL);

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

cout << fairRations(B) << "\n";

return 0;

}