#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 10;
const ll INFL = 4e18;

const int mod = 998244353;
const int mx = 1e8 + 5;
int fact[mx], factinv[mx];

int modpow(ll a, int b) {
    ll ret = 1;
    while (b) {
        if (b & 1) (ret *= a) %= mod;
        (a *= a) %= mod;
        b /= 2;
    }
    return ret;
}

int C(int a, int b) {
    if (a < b) return 0;
    if (a < 0 || b < 0) return 0;
    ll ret = fact[a];
    ret *= factinv[b];
    ret %= mod;
    ret *= factinv[a - b];
    ret %= mod;
    return ret;
}

int main() {
    int K;
    cin >> K;
    vector<int> L(K), M(K);
    int N = 0;
    for (int i = 0; i < K; i++) {
        cin >> L[i] >> M[i];
        N += L[i] * M[i];
    }

    ll tmp;
    fact[0] = 1;
    for (int i = 0; i < N; i++) {
        tmp = fact[i];
        tmp *= i + 1;
        tmp %= mod;
        fact[i + 1] = tmp;
    }
    factinv[N] = modpow(fact[N], mod - 2);
    for (int i = N; i > 0; i--) {
        tmp = factinv[i];
        tmp *= i;
        tmp %= mod;
        factinv[i - 1] = tmp;
    }

    ll ans = fact[N];
    for (int i = 0; i < K; i++) {
        ans *= factinv[L[i] * M[i]];
        ans %= mod;
    }

    for (int i = 0; i < K; i++) {
        int base = L[i];
        for (int t = 0; t < M[i] - 1; t++) {
            int add = L[i] - 1;
            base += add;
            ans *= C(base, add);
            ans %= mod;
            base++;
        }
    }

    cout << ans << endl;
}