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

template <int MOD_> struct modnum {
	static constexpr int MOD = MOD_;
	static_assert(MOD_ > 0, "MOD must be positive");

private:
	using ll = long long;

	int v;

	static int minv(int a, int m) {
		a %= m;
		assert(a);
		return a == 1 ? 1 : int(m - ll(minv(m, a)) * ll(m) / a);
	}

public:

	modnum() : v(0) {}
	modnum(ll v_) : v(int(v_ % MOD)) { if (v < 0) v += MOD; }
	explicit operator int() const { return v; }
	friend std::ostream& operator << (std::ostream& out, const modnum& n) { return out << int(n); }
	friend std::istream& operator >> (std::istream& in, modnum& n) { ll v_; in >> v_; n = modnum(v_); return in; }

	friend bool operator == (const modnum& a, const modnum& b) { return a.v == b.v; }
	friend bool operator != (const modnum& a, const modnum& b) { return a.v != b.v; }

	modnum inv() const {
		modnum res;
		res.v = minv(v, MOD);
		return res;
	}
	modnum neg() const {
		modnum res;
		res.v = v ? MOD-v : 0;
		return res;
	}

	modnum operator- () const {
		return neg();
	}
	modnum operator+ () const {
		return modnum(*this);
	}

	modnum& operator ++ () {
		v ++;
		if (v == MOD) v = 0;
		return *this;
	}
	modnum& operator -- () {
		if (v == 0) v = MOD;
		v --;
		return *this;
	}
	modnum& operator += (const modnum& o) {
		v += o.v;
		if (v >= MOD) v -= MOD;
		return *this;
	}
	modnum& operator -= (const modnum& o) {
		v -= o.v;
		if (v < 0) v += MOD;
		return *this;
	}
	modnum& operator *= (const modnum& o) {
		v = int(ll(v) * ll(o.v) % MOD);
		return *this;
	}
	modnum& operator /= (const modnum& o) {
		return *this *= o.inv();
	}

	friend modnum operator ++ (modnum& a, int) { modnum r = a; ++a; return r; }
	friend modnum operator -- (modnum& a, int) { modnum r = a; --a; return r; }
	friend modnum operator + (const modnum& a, const modnum& b) { return modnum(a) += b; }
	friend modnum operator - (const modnum& a, const modnum& b) { return modnum(a) -= b; }
	friend modnum operator * (const modnum& a, const modnum& b) { return modnum(a) *= b; }
	friend modnum operator / (const modnum& a, const modnum& b) { return modnum(a) /= b; }
};
using num = modnum<998244353>;

const int MAXV = 6000;
num fact[MAXV];
num ifact[MAXV];

const int MAXN = 500;
const int MAXSA = 500;
const int MAXSB = 500;
int N;
int SA;
int A[MAXN];
int SB;
int B[MAXN];

num dp[MAXSA][MAXSB];

int main() {
	ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

	fact[0] = 1;
	for (int i = 1; i < MAXV; i++) {
		fact[i] = fact[i-1] * num(i);
	}
	ifact[MAXV-1] = fact[MAXV-1].inv();
	for (int i = MAXV - 1; i > 0; i--) {
		ifact[i-1] = ifact[i] * num(i);
	}

	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> A[i] >> B[i];
	}

	dp[0][0] = 1;
	for (int i = 0; i < N; i++) {
		for (int sa = SA; sa >= 0; sa--) {
			for (int sb = 0; sb <= SB; sb++) {
				num apow = 1;
				for (int b = 0; b < B[i]; b++, apow *= A[i]) {
					dp[sa+A[i]][sb+b] += num(-1) * dp[sa][sb] * fact[sb + b] * ifact[sb] * ifact[b] * apow;
				}
			}
		}
		SA += A[i];
		SB += B[i];
	}

	num ans = 0;
	for (int sa = 1; sa <= SA; sa++) {
		num den = 1;
		for (int sb = 0; sb <= SB; sb++, den *= sa) {
			dp[sa][sb] /= den;
			ans += dp[sa][sb] * num(SA) / num(sa);
		}
	}
	cout << -ans << '\n';

	return 0;
}


Measured memory_bytes: 1280