#include <atcoder/modint>
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using mint = atcoder::modint998244353;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  ll k;
  cin >> k;
  ll rem = 0;
  vector<ll> ls(k), ms(k);
  rep(i, k) {
    ll l, m;
    cin >> l >> m;
    ls[i] = l, ms[i] = m, rem += l * m;
  }
  mint ans = 1;
  rep(ki, k) {
    ll l = ls[ki], m = ms[ki];
    mint now1 = 1, now2 = 1;
    rep(i, l * m) {
      now1 *= rem - i;
      now2 *= i + 1;
    }
    rep(mi, m) {
      rep(li, l - 1) {
        now1 *= (m - mi) * l - li - 1;
        now2 *= li + 1;
      }
    }
    rem -= l * m;
    ans *= now1;
    ans /= now2;
  }
  cout << ans.val() << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}