#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;
	}
	friend modnum inv(const modnum& m) { return m.inv(); }
	modnum neg() const {
		modnum res;
		res.v = v ? MOD-v : 0;
		return res;
	}
	friend modnum neg(const modnum& m) { return m.neg(); }

	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; }
};

template <typename T> T pow(T a, long long b) {
	assert(b >= 0);
	T r = 1; while (b) { if (b & 1) r *= a; b >>= 1; a *= a; } return r;
}

using num = modnum<998244353>;

int main() {
	ios::sync_with_stdio(0), cin.tie(0);
	int n;
	cin >> n;
	vector<int> A(n);
	vector<int> B(n);
	for (int i = 0; i < n; i++) {
		cin >> A[i] >> B[i];
	}

	const int M = 410;

	vector<num> fact(M);
	fact[0] = 1;
	for (int i = 1; i < M; i++) fact[i] = fact[i-1] * i;
	vector<num> ifact(M);
	for (int i = 0; i < M; i++) ifact[i] = 1 / fact[i];

	vector<vector<num>> dp(M, vector<num>(M));
	dp[0][0] = -1;
	int ta = 0, tb = 0;
	for (int i = 0; i < n; i++) {
		for (int a = ta; a >= 0; a--) {
			for (int b = 0; b <= tb; b++) {
				num apow = 1;
				for (int w = 0; w < B[i]; w++) {
					dp[a + A[i]][b + w] -= dp[a][b] * ifact[w] * apow;
					apow *= A[i];
				}
			}
		}
		ta += A[i];
		tb += B[i];
	}
	num ans = 0;
	for (int a = 1; a <= ta; a++) {
		num inva = 1 / num(a);
		num denom = 1;
		for (int b = 0; b <= tb; b++) {
			ans += dp[a][b] * fact[b] * denom * ta * inva;
			denom *= inva;
		}
	}
	cout << ans << '\n';
}


Measured memory_bytes: 896