cf 1011C. Fly

二分，每次判断经过所有星球是否把油耗光

#include<bits/stdc++.h>
using namespace std;
int a[1010];
int b[1010];
int n,m;
bool check(double x)
{
    double weight=x+m;
    double tot=x;
    for(int i=1;i<=n;i++)
    {
        if(i==1)
        {
            double tmp=weight/a[i];
            weight-=tmp;
            tot-=tmp;
            if(tot<0)
              return false;
        }
        else if(i==n)
        {
            double tmp1=weight/b[i];
            weight=weight-tmp1;
            tot-=tmp1;
            if(tot<0)
              return false;
            double tmp2=weight/a[i];
            weight=weight-tmp2;
            tot-=tmp2;
            if(tot<0)
              return false;
            double tmp3=weight/b[1];
            weight=weight-tmp3;
            tot-=tmp3;
            if(tot<0)
              return false;
        }
        else
        {
            double tmp1=weight/b[i];
            weight=weight-tmp1;
            tot-=tmp1;
            if(tot<0)
              return false;
            double tmp2=weight/a[i];
            weight=weight-tmp2;
            tot-=tmp2;
            if(tot<0)
              return false;
        }
    }
    return true;
}
int main()
{
    scanf("%d",&n);
    scanf("%d",&m);
    for(int i=1;i<=n;i++)
       scanf("%d",&a[i]);
    for(int i=1;i<=n;i++)
       scanf("%d",&b[i]);
    double le=0,ri=1e9;
    double ans=-1;
    for(int i=1;i<=100;i++)
    {
        double mid=(le+ri)/2;
        if(check(mid))
        {
            ri=mid;
            ans=mid;
        }
        else
        {
            le=mid;
        }
    }
    if(ans>0)
    printf("%.10lf\n",ans);
    else
    printf("-1\n");
}