#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cstring>
#include <cassert>
#include <algorithm>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <tuple>
#include <queue>
#include <deque>
#include <list>
#define f0(_X,_Y) for(int (_X)=0;(_X)<(_Y);++(_X))
#define f1(_X,_Y) for(int (_X)=1;(_X)<=(_Y);++(_X))
#define ff(_X,_Y,_Z) for(int (_X)=(_Y);(_X)<=(_Z);++(_X))
#define fF(_X,_Y,_Z) for(int (_X)=(_Y);(_X)<(_Z);++(_X))
#define rf0(_X,_Y) for(int _X=(_Y)-1;(_X)>=0;--(_X))
#define rf1(_X,_Y) for(int _X=(_Y);(_X)>0;--(_X))
#define rff(_X,_Y,_Z) for(int _X=(_Y);(_X)>=(_Z);--(_X))
#define rfF(_X,_Y,_Z) for(int _X=(_Y);(_X)>(_Z);--(_X))
#define PRT(_X) cout<< #_X ": "<<_X<<endl;
#define TIME fprintf(stderr,"time : %.2f sec\n",double(clock())/CLOCKS_PER_SEC)
#define FIN freopen("input","r",stdin)
#define scan1(_X) scanf("%d",&_X);
#define scan2(_X,_Y) scanf("%d%d",&_X,&_Y);
#define scan3(_X,_Y,_Z) scanf("%d%d%d",&_X,&_Y,&_Z);
#define define1(_1) int _1; scan1(_1)
#define define2(_1,_2) int _1,_2; scan2(_1,_2)
#define define3(_1,_2,_3) int _1,_2,_3; scan3(_1,_2,_3)
#define EXPAND(_1) _1
#define SELECT(_1,_2,_3,_4,NAME,...) NAME
#define scan(...) EXPAND(SELECT(__VA_ARGS__, scan4, scan3, scan2, scan1)(__VA_ARGS__))
#define define(...) EXPAND(SELECT(__VA_ARGS__, define4, define3, define2, define1)(__VA_ARGS__))
#define print(_X) printf("%d\n",_X)
#define PAIR_STRUCT(_T,_X,_Y,...) struct _T{int _X,_Y,##__VA_ARGS__; bool friend operator < (const _T &p, const _T &q){if(p._X!=q._X) return p._X<q._X; return p._Y<q._Y;}}
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
const int N = 5005;
int p[N],pR[N];
int d[N],d2[N];
const int MOD = 1000000007;
ll mul(ll a, ll b){return a*b%MOD;}
ll add(ll a, ll b){return (a+b)%MOD;}
ll inv(ll a, ll b=MOD)
{
ll s = 0; ll old_s = 1;
ll r = b; ll old_r = a;
while (r!=0)
{
ll quotient = old_r / r;
tie(old_r, r) = make_tuple(r, old_r - quotient * r);
tie(old_s, s) = make_tuple(s, old_s - quotient * s);
}
ll inv = old_s%MOD;
if(inv<0) inv += MOD;
return inv;
}
int dv[N];
int deal[N];
int _inv[N];
int main()
{
// divisors
ff(i,2,5000)
{
for(int j=i;j<=5000;j+=i) if(!dv[j]) dv[j]=i;
}
// inverse
_inv[1]=1;
ff(i,2,5000)
{
if(dv[i]!=i)
_inv[i]= mul(_inv[dv[i]],_inv[i/dv[i]]);
else
_inv[i]= inv(i);
}
define(n);
const int denominator = 1000000000;
const ll denominator_inv = inv(denominator);
f1(i,n) { scan(p[i],deal[i]); pR[i] = mul(denominator-p[i],denominator_inv); p[i]=mul(p[i],denominator_inv); }
d[0]=1;
f1(i,n) rf1(j,i) d[j] = add(d[j],mul(pR[i],d[j-1]));
ll ans=0;
f1(i,n)
{
ll nCm_inv = _inv[n];
// 1/nCm = 1*2*..*m/(n*(n-1)..(n-m+1))
ll dmg = 0;
ff(m,0,n-1)
{
if(!m)
d2[m]=1;
else
{
d2[m] = add(d[m],MOD - mul(d2[m-1],pR[i]));
nCm_inv = mul(mul(nCm_inv, m), _inv[n-m]);
}
dmg = add(dmg,mul(d2[m],nCm_inv));
}
ans = add(ans, mul(dmg,mul(p[i],deal[i])));
}
printf("%lld\n",ans);
return 0;
}
