#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	int N;
	cin>>N;
	
	vector<int> a(N-1);
	rep(i,N-1)cin>>a[i];
	double v = 1.0;
	rep(i,N-1){
		v *= 1000.0 - a[i];
		v /= 1000.0;
	}
	
	double ans = 1000.0;
	ans -= 1000.0 * v;
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
    return 0;
}