#include <cstdio>
const int M = 405;
const int MOD = 998244353;
#define int long long
#define rep(i,s,t) for(int i=(s);i<=(t);i++) 
int read()
{
	int x=0,f=1;char c;
	while((c=getchar())<'0' || c>'9') {if(c=='-') f=-1;}
	while(c>='0' && c<='9') {x=(x<<3)+(x<<1)+(c^48);c=getchar();}
	return x*f;
}
int n,m,a[M],b[M],fac[M],inv[M],finv[M];
int ans,m1,m2,pw[M][M],dp[M][M][M];
void init(int n)
{
	inv[0]=inv[1]=finv[0]=fac[0]=1;
	rep(i,1,n) fac[i]=fac[i-1]*i%MOD;
	rep(i,2,n) inv[i]=inv[MOD%i]*(MOD-MOD/i)%MOD;
	rep(i,1,n) finv[i]=finv[i-1]*inv[i]%MOD;
}
signed main()
{
	n=read();init(m=400);
	rep(i,1,n)
	{
		m1+=a[i]=read();m2+=b[i]=read();pw[i][0]=1;
		rep(j,1,b[i]) pw[i][j]=pw[i][j-1]*a[i]%MOD;
	}
	dp[0][0][0]=-1;
	rep(i,1,n) rep(j,0,m1) rep(k,0,m2)
	{
		dp[i][j][k]=dp[i-1][j][k];
		rep(l,0,b[i]-1) if(a[i]<=j && l<=k)
			dp[i][j][k]=(dp[i][j][k]-dp[i-1][j-a[i]][k-l]
			*pw[i][l]%MOD*finv[l])%MOD;
	}
	rep(j,1,m1)
	{
		int x=m1*inv[j]%MOD;
		rep(k,0,m2)
		{
			ans=(ans+x*fac[k]%MOD*dp[n][j][k])%MOD;
			x=x*inv[j]%MOD;
		}
	}
	printf("%lld\n",(ans+MOD)%MOD);
}