#include<stdio.h>
long long BS = 1000000000, Mod = 1000000007;
int n;
long long P[5010][5010], D[5010][5010], C[5010], Succ[5010], Fail[5010], Inv[5010];
long long Pow(long long a, long long b){
long long r = 1;
while(b){
if(b%2)r=r*a%Mod;
a=a*a%Mod;b/=2;
}
return r;
}
long long Invv(long long a){
return Pow(a,Mod-2);
}
int main(){
int i, j;
scanf("%d",&n);
for(i=1;i<=n;i++){
scanf("%lld %lld",&Succ[i],&C[i]);
Fail[i] = BS-Succ[i];
Inv[i] = Invv(i);
}
P[0][0]=1;
long long SS;
for(i=1;i<=n;i++){
P[i][0]=P[i-1][0]*BS%Mod;
for(j=1;j<=i;j++){
P[i][j] = (P[i-1][j]*(i-j)%Mod*Inv[i]%Mod*BS%Mod + P[i-1][j-1]*j%Mod*Inv[i]%Mod*Fail[i]%Mod)%Mod;
}
for(j=0;j<i;j++){
D[i][j+1] = (Fail[i]%Mod*D[i-1][j]%Mod*j%Mod + BS*D[i-1][j+1]%Mod*(i-j-1)%Mod + Succ[i]*C[i]%Mod*P[i-1][j])%Mod;
D[i][j+1] = D[i][j+1]*Inv[i]%Mod;
}
}
SS = 0;
for(i=1;i<=n;i++)SS = (SS+D[n][i])%Mod;
printf("%lld\n",SS*Pow(Invv(BS),n)%Mod);
}
