#include <bits/stdc++.h>
using namespace std;
#define int long long
const int mod = 998244353;
const int maxn = 1e5 + 5;
int n, x[maxn], y[maxn], p[maxn];

int fpow(int a, int b) {
	int ans = 1, t = a;
	while (b) {
		if (b & 1)
			ans = ans * t % mod;
		t = t * t % mod;
		b >>= 1;
	}
	return ans;
}

int inv(int a) {
	return fpow(a, mod-2);
}

signed main() {
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> x[i] >> y[i];
		p[i] = x[i] * inv(y[i]) % mod;
	}
	int x = 0, y = 0;
	for (int i = 1, t = 1; i <= n; i++) {
		x += t * p[i] % mod;
		x %= mod;
		y += t;
		y %= mod;
		t = t * (1 - p[i] + mod) % mod;
	}
	cout << y * inv((1 - x + mod) % mod) % mod << endl;
	return 0;
}

https://www.cnblogs.com/quanjun/p/18411000