#define MOD_TYPE 2 #pragma region Macros #include using namespace std; #if 0 #include #include using Int = boost::multiprecision::cpp_int; using lld = boost::multiprecision::cpp_dec_float_100; #endif #pragma GCC target("avx2") #pragma GCC optimize("O3") #pragma GCC optimize("unroll-loops") using ll = long long int; using ld = long double; using pii = pair; using pll = pair; using pld = pair; template using smaller_queue = priority_queue, greater>; constexpr ll MOD = (MOD_TYPE == 1 ? (ll)(1e9 + 7) : 998244353); //constexpr ll MOD = 1; constexpr int INF = (int)1e9 + 10; constexpr ll LINF = (ll)4e18; constexpr double PI = acos(-1.0); constexpr double EPS = 1e-11; constexpr int Dx[] = {0, 0, -1, 1, -1, 1, -1, 1, 0}; constexpr int Dy[] = {1, -1, 0, 0, -1, -1, 1, 1, 0}; #define REP(i, m, n) for (ll i = m; i < (ll)(n); ++i) #define rep(i, n) REP(i, 0, n) #define REPI(i, m, n) for (int i = m; i < (int)(n); ++i) #define repi(i, n) REPI(i, 0, n) #define MP make_pair #define MT make_tuple #define YES(n) cout << ((n) ? "YES" : "NO") << "\n" #define Yes(n) cout << ((n) ? "Yes" : "No") << "\n" #define possible(n) cout << ((n) ? "possible" : "impossible") << "\n" #define Possible(n) cout << ((n) ? "Possible" : "Impossible") << "\n" #define Yay(n) cout << ((n) ? "Yay!" : ":(") << "\n" #define all(v) v.begin(), v.end() #define NP(v) next_permutation(all(v)) #define dbg(x) cerr << #x << ":" << x << "\n"; struct io_init { io_init() { cin.tie(0); ios::sync_with_stdio(false); cout << setprecision(30) << setiosflags(ios::fixed); }; } io_init; template inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; } template inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; } inline ll CEIL(ll a, ll b) { return (a + b - 1) / b; } template inline void Fill(A (&array)[N], const T &val) { fill((T *)array, (T *)(array + N), val); } template constexpr istream &operator>>(istream &is, pair &p) noexcept { is >> p.first >> p.second; return is; } template constexpr ostream &operator<<(ostream &os, pair &p) noexcept { os << p.first << " " << p.second; return os; } #pragma endregion #pragma region mint template struct Fp { long long val; constexpr Fp(long long v = 0) noexcept : val(v % MOD) { if (val < 0) v += MOD; } constexpr int getmod() { return MOD; } constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; } constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; } constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; } constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; } constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; } constexpr Fp &operator+=(const Fp &r) noexcept { val += r.val; if (val >= MOD) val -= MOD; return *this; } constexpr Fp &operator-=(const Fp &r) noexcept { val -= r.val; if (val < 0) val += MOD; return *this; } constexpr Fp &operator*=(const Fp &r) noexcept { val = val * r.val % MOD; if (val < 0) val += MOD; return *this; } constexpr Fp &operator/=(const Fp &r) noexcept { long long a = r.val, b = MOD, u = 1, v = 0; while (b) { long long t = a / b; a -= t * b; swap(a, b); u -= t * v; swap(u, v); } val = val * u % MOD; if (val < 0) val += MOD; return *this; } constexpr bool operator==(const Fp &r) const noexcept { return this->val == r.val; } constexpr bool operator!=(const Fp &r) const noexcept { return this->val != r.val; } friend constexpr ostream &operator<<(ostream &os, const Fp &x) noexcept { return os << x.val; } friend constexpr istream &operator>>(istream &is, Fp &x) noexcept { return is >> x.val; } }; Fp modpow(const Fp &a, long long n) noexcept { if (n == 0) return 1; auto t = modpow(a, n / 2); t = t * t; if (n & 1) t = t * a; return t; } using mint = Fp; #pragma endregion void solve() { int n; cin >> n; if (n == 1) { int a; cin >> a; cout << (a == -2) << "\n"; return; } int a[200010]; a[0] = a[n + 1] = 0; rep(i, n) cin >> a[i + 1]; vector pos; rep(i, n + 2) { if (abs(a[i]) != 1) pos.push_back(i); } mint sum = 0; rep(pi, pos.size()) { if (a[pos[pi]] == 0) continue; mint cntR[2] = {}, cntL[2] = {}; bool parity = 0; for (int i = pos[pi], j = 0; i < pos[pi + 1]; i++, j++) { if (a[i] == -1) parity ^= 1; if (i == n) cntR[parity] += mint(1) / modpow(2, j) * 2; else cntR[parity] += mint(1) / modpow(2, j); } parity = 0; for (int i = pos[pi], j = 0; i > pos[pi - 1]; i--, j++) { if (a[i] == -1) parity ^= 1; if (i == 1) cntL[parity] += mint(1) / modpow(2, j) * 2; else cntL[parity] += mint(1) / modpow(2, j); } mint t; if (a[pos[pi]] == 2) { t = (cntL[0] * cntR[1] + cntL[1] * cntR[0]) * modpow(2, n - 3); } else { t = (cntL[0] * cntR[0] + cntL[1] * cntR[1]) * modpow(2, n - 3); } sum += t; } cout << sum / modpow(2, n - 1) << "\n"; } int main() { solve(); }