#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

template<unsigned P> struct ModInt {
  using M = ModInt;
  unsigned v;
  constexpr ModInt() : v(0) {}
  constexpr ModInt(auto x) : v(x >= 0 ? x % P : (P - -x % P) % P) {}
  constexpr ModInt(unsigned v, int) : v(v) {}
  static constexpr unsigned p() { return P; }
  M operator+() const { return *this; }
  M operator-() const { return {v ? P - v : 0, 0}; }
  explicit operator bool() const noexcept { return v; }
  bool operator!() const noexcept { return !(bool)*this; }
  M operator*(M r) const { return M(*this) *= r; }
  M operator/(M r) const { return M(*this) /= r; }
  M operator+(M r) const { return M(*this) += r; }
  M operator-(M r) const { return M(*this) -= r; }
  bool operator==(M r) const { return v == r.v; }
  bool operator!=(M r) const { return !(*this == r); }
  M& operator*=(M r) { v = (uint64_t)v * r.v % P; return *this; }
  M& operator/=(M r) { return *this *= r.inv(); }
  M& operator+=(M r) { if ((v += r.v) >= P) v -= P; return *this; }
  M& operator-=(M r) { if ((v += P - r.v) >= P) v -= P; return *this; }
  M inv() const {
    int a = v, b = P, x = 1, u = 0;
    while (b) {
      int q = a / b;
      swap(a -= q * b, b);
      swap(x -= q * u, u);
    }
    assert(a == 1);
    return x;
  }
  M pow(auto n) const {
    if (n < 0) return pow(-n).inv();
    M res = 1;
    for (M a = *this; n; a *= a, n >>= 1) if (n & 1) res *= a;
    return res;
  }
  friend M operator*(auto l, M r) { return M(l) *= r; }
  friend M operator/(auto l, M r) { return M(l) /= r; }
  friend M operator+(auto l, M r) { return M(l) += r; }
  friend M operator-(auto l, M r) { return M(l) -= r; }
  friend ostream& operator<<(ostream& os, M r) { return os << r.v; }
  friend istream& operator>>(istream& is, M& r) { lint x; is >> x; r = x; return is; }
  friend bool operator==(auto l, M r) { return M(l) == r; }
  friend bool operator!=(auto l, M r) { return !(l == r); }
};
using Mint = ModInt<998244353>;

V<Mint> fact, ifact;
void init(int n, int B = 2) {
  fact.resize(n + 1);
  fact[0] = 1;
  for (int i = 1; i <= n; ++i) {
    fact[i] = i * fact[i - 1];
  }
  ifact.resize(n + 1);
  ifact[n] = 1 / fact[n];
  for (int i = n; i > 0; --i) {
    ifact[i - 1] = i * ifact[i];
  }
}

int main() {

  cin.tie(nullptr); ios::sync_with_stdio(false);
  int n; cin >> n;
  V<> a(n), b(n); for (int i = 0; i < n; ++i) cin >> a[i] >> b[i];
  int s = accumulate(begin(a), end(a), 0);
  int t = accumulate(begin(b), end(b), 0) - n;
  init(t);

  V< VV<Mint> > dp(2, VV<Mint>(s + 1, V<Mint>(t + 1)));
  dp[0][0][0] = 1;
  for (int i = 0; i < n; ++i) {
    V<Mint> powa(b[i], 1);
    for (int j = 1; j < b[i]; ++j) {
      powa[j] = powa[j - 1] * a[i];
    }
    auto ndp = dp;
    for (int p = 0; p < 2; ++p) for (int sa = 0; sa <= s; ++sa) for (int sj = 0; sj <= t; ++sj) if (dp[p][sa][sj]) {
      for (int j = 0; j < b[i]; ++j) {
        ndp[p ^ 1][sa + a[i]][sj + j] += dp[p][sa][sj] * powa[j] * ifact[j];
      }
    }
    swap(dp, ndp);
    for (int p = 0; p < 2; ++p) for (int sa = 0; sa <= s; ++sa) for (int sj = 0; sj <= t; ++sj) if (dp[p][sa][sj]) {
    }
  }

  Mint res;
  for (int sa = 1; sa <= s; ++sa) for (int sj = 0; sj <= t; ++sj) {
    res -= fact[sj] / Mint(sa).pow(sj + 1) * dp[0][sa][sj];
    res += fact[sj] / Mint(sa).pow(sj + 1) * dp[1][sa][sj];
  }
  res *= accumulate(begin(a), end(a), 0);
  cout << res << '\n';
}

Measured memory_bytes: 2824