#include<iostream>
#include<cstdio>
#include<vector>
#define N 50000
using namespace std;
long long read()
{
    char c=0;
    long long sum=0;
    while (c<'0'||c>'9') c=getchar();
    while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
    return sum;
}
__int128 T,a,b,sx,sy,n;
double ans;
__int128 gcd(__int128 a,__int128 b)
{
    if (!b) return a;
    return gcd(b,a%b);
}
__int128 lcm(__int128 a,__int128 b)
{
    return a/gcd(a,b)*b;
}
void exgcd(__int128 a,__int128 b)
{
    if (!b)
    {
	sx=1,sy=0;
	return;
    }
    exgcd(b,a%b);
    __int128 tx=sx;
    sx=sy,sy=tx-(a/b)*sy;
    return;
}
double solve(vector<double>v)
{
    if (v.size()==2) return max(v[0],v[1]);
    double res=0,rest=0,maxn;
    int cnt=0;
    vector<double>sv;
    for (int i=0;i<v.size();++i) res=max(res,v[i]);
    for (int i=0;i<v.size();++i)
	if (v[i]==res)
	    cnt++;
    for (int i=0;i<v.size();++i)
	if (v[i]==res)
	{
	    maxn=0;
	    for (int j=1;j<v.size();++j)
		if (i!=j)
		{
		    sv.clear();
		    for (int k=0;k<v.size();++k)
			if (k!=j)
			{
			    if (k==i) sv.push_back(v[k]);
			    else sv.push_back(v[k]*(1-v[i])/(1-v[i]-v[j]));
			}
		    maxn=max(maxn,solve(sv));
		}
	    rest+=maxn/cnt;
	}
    return rest;
}
int main()
{
    vector<double>v;
    __int128 d=0,p=1,sp,g;
    T=read();
    while (T--)
    {
	a=read(),b=read(),g=gcd(p,b),sp=lcm(p,b);
	if ((a-d)%g!=0)
	{
	    puts("error");
	    return 0;
	}
	exgcd(p,b),sx=((sx*((a-d)/g))%sp+sp)%sp,d=(p*sx+d)%sp,p=sp;
    }
    n=d;
    if (n<2)
    {
	puts("error");
	return 0;
    }
    for (int i=1;i<=n;++i) v.push_back(1.0/n);
    printf("%0.6lf\n",solve(v));
    return 0;
}