#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
using namespace std;
typedef long long LL;
const int maxn=410;
const int mod=998244353;
inline int ad(int x,int y){return x>=mod-y?x-mod+y:x+y;}
inline int re(int x,int y){return x<y?x-y+mod:x-y;}
inline int mu(int x,int y){return (LL)x*y%mod;}
inline int qp(int x,LL y){if(y==-1)y=mod-2;int r=1;while(y){if(y&1)r=mu(r,x);x=mu(x,x);y>>=1;}return r;}
inline int iv(int x){return qp(x,mod-2);}
inline int dv(int x,int y){return mu(x,qp(y,mod-2));}

int fac[maxn],fac_inv[maxn];
int a[maxn],b[maxn],f[maxn][maxn];
int main()
{
	fac[0]=1;for(int i=1;i<maxn;i++)fac[i]=mu(fac[i-1],i);
	fac_inv[maxn-1]=iv(fac[maxn-1]);
	for(int i=maxn-2;i>=0;i--)fac_inv[i]=mu(fac_inv[i+1],i+1);
	int n,A=0,tA=0,B=0;
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%d%d",&a[i],&b[i]),A+=a[i],B+=b[i]-1;
	tA=iv(A);
	
	f[0][0]=1;
	for(int s=1,suma=a[1],sumb=b[1]-1;s<=n;s++,suma+=a[s],sumb+=b[s]-1)
		for(int i=suma;i>=a[s];i--)
			for(int j=sumb;j>=0;j--)
				for(int t=0;t<b[s]&&j>=t;t++)
					f[i][j]=ad(f[i][j],mu(mu(re(0,qp(mu(a[s],tA),t)),fac_inv[t]),f[i-a[s]][j-t]));
	
	int ans=0;
	for(int i=1;i<=A;i++)
		for(int j=0;j<=B;j++)
			ans=ad(ans,mu(dv(fac[j],qp(mu(i,tA),j+1)),re(0,f[i][j])));
	printf("%d\n",ans);

	return 0;
}
