#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>
#include <tuple>
#include <bitset>
#include <map>

#define rep(i,n) for(i=0; i<n; ++i)
#define inc_rep(i,a,n) for(i=a; i<=n; ++i)
#define dec_rep(i,n,a) for(i=n; i>=a; --i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
#define print_vec(v) for(auto it=v.begin();it!=v.end();++it)cout<<*it <<" ";cout<<endl
#define print_vec2d(v) for(int i=0;i<v.size();++i){for(int j=0;j<v[i].size();++j)cout<<v[i][j]<<" ";cout<<endl;}cout<<endl
using namespace std;
using lint = long long;


int main(void){ 
    int i, j, k, cnt=0, x, y;
    lint p, q, n, m;
    string s;
    in(n);
    vector<int> v(n-1);
    rep(i,n-1) in(v[i]);
    double a=1000.0;
    rep(i,n-1) a*=((1000.0-v[i])/1000.0);
    a=1000.0-a;
    printf("%.25f\n", a);
    return 0;
}