package main

import . "fmt"
import . "math/big"

const M = 998244353

func main() {
	var k int
	Scan(&k)
	var num, den, x int64 = 1,1,1
	for i := 0; i < k; i++ {
		var l int64
		var m int
		Scan(&l,&m)
		y := l
		for j := 0; j < m; j++ {
			for z := int64(1); z < l; z++ {
				num = (num*x)%M
				den = (den*z)%M
				x++
			}
			num = (num*x)%M
			den = (den*y)%M
			x++
			y += l
		}
	}
	den = new(Int).ModInverse(NewInt(den), NewInt(M)).Int64()
	Println(num*den%M)
}