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#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<set>
#include<bitset>
#include<complex>
#include<cstdlib>
#include<assert.h>
#define inc(i,l,r) for(int i=l;i<=r;i++)
#define dec(i,l,r) for(int i=l;i>=r;i--)
#define link(x) for(edge *j=h[x];j;j=j->next)
#define mem(a) memset(a,0,sizeof(a))
#define ll long long
#define eps 1e-8
#define succ(x) (1<<x)
#define lowbit(x) (x&(-x))
#define mid (x+y>>1)
#define sqr(x) ((x)*(x))
#define NM 1100005
#define nm 400005
using namespace std;
const double pi=acos(-1);
const ll inf=998244353;
ll read(){
ll x=0,f=1;char ch=getchar();
while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
while(isdigit(ch))x=x*10+ch-'0',ch=getchar();
return f*x;
}
int n,tot;
double ans,a[NM];
int main(){
while(~scanf("%d",&n)){
inc(i,1,n)scanf("%lf",a+i);
tot=succ(n)-1;
ans=0;
inc(i,1,tot){
int cnt=0;double t=0;
inc(j,1,n)if(succ(j-1)&i)t+=a[j],cnt++;
t=1/t;if(cnt&1)ans+=t;else ans-=t;
}
printf("%lf\n",ans);
}
return 0;
}
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