#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;

#define For(i, a, b) for(int i = (a); i < (b); i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(int i = (a); i >= (b); i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()

using lint = long long;
using ld = long double;

int INF = 2000000000;
lint LINF = 1000000000000000000;

using mint = atcoder::modint998244353;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int k;
    cin >> k;
    vector<int> l(k), m(k);
    int n = 0;
    rep(i, k) {
        cin >> l[i] >> m[i];
        n += l[i] * m[i];
    }
    
    set<int> num;
    int sum = n;
    rep(i, k) {
        num.insert(sum);
        num.insert(l[i] * m[i]);
        num.insert(sum - l[i] * m[i]);
        sum -= l[i] * m[i];
    }
    rep(i, k) {
        int rest = l[i] * m[i];
        rep(_, m[i]) {
            num.insert(rest);
            num.insert(l[i]);
            num.insert(rest - l[i]);
            rest -= l[i];
        }
        num.insert(m[i]);
    }
    map<int, mint> Fac, Finv;
    Fac[0] = Fac[1] = 1;
    Finv[0] = Finv[1] = 1;
    mint now = 1;
    for (int i = 2; i <= n; i++) {
        now *= mint(i);
        if (num.count(i)) {
            Fac[i] = now;
            Finv[i] = now.inv();
        }
    }
    
    auto comb = [&](int n, int k) -> mint {
        return Fac[n] * Finv[k] * Finv[n - k];
    };
    
    mint ans = 1;
    sum = n;
    rep(i, k) {
        ans *= comb(sum, l[i] * m[i]);
        sum -= l[i] * m[i];
    }
    rep(i, k) {
        int rest = l[i] * m[i];
        rep(_, m[i]) {
            ans *= comb(rest, l[i]);
            rest -= l[i];
        }
        ans /= Fac[m[i]];
    }
    cout << ans.val() << "\n";
}