#include <bits/stdc++.h>
using namespace std;
long long Qread()
{
    long long x=0;char ch=getchar();
    while(ch<'0'||ch>'9') ch=getchar();
    while(ch>='0'&&ch<='9') x=x*10+(ch^48),ch=getchar();
    return x;
}
void exgcd(long long a,long long b,__int128 &x,__int128 &y,long long &gcd)//ax+by=gcd
{
    if(b==0)
    {
        x=1,y=0,gcd=a;
        return;
    }
    exgcd(b,a%b,x,y,gcd);
    x-=a/b*y;
    swap(x,y);
    while(y<0) x-=b/gcd,y+=a/gcd;
    while(y>=a/gcd) x+=b/gcd,y-=a/gcd;
    return;
}
int Q;
long long A,A_,tgcd;
__int128 tx,ty,x,x_;
long long get_n()
{
    Q=Qread();x=0,A=1;
    while(Q--)
    {
        x_=Qread(),A_=Qread();
        exgcd(A,A_,tx,ty,tgcd);
        if((x-x_)%tgcd)
        {
            printf("error\n");
            exit(0);
        }
        ty*=(x-x_)/tgcd;
        A=A*A_/tgcd;
        x=(x_+ty*A_)%A;
        if(x<0) x+=A; 
    }
    if(x<2)
    {
        printf("error\n");
        exit(0);
    }
    return (long long)x;
}
long long n;
double p[]={1.000000,1.000000,0.500000,0.666667,0.625000,0.466667,0.416667,0.342857,0.291667,0.253968,0.225000};
int main()
{
    n=get_n();
    if(n>10) printf("0.000000\n");
    else printf("%.6lf\n",p[n]);
    return 0;
}