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*/
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll>pll;
/* Nothing is ture, (22)
Everything is permitted*/
#define fi  first
#define se second
#define pb(x)       push_back(x)
#define mp(a,b)  make_pair(a,b)
#define sz(a)  ((int)(a).size())
#define all(x) x.begin(),x.end()
#define msort(a) sort(a+1,a+1+n);
#define endl "\n"

#define ff(i,a,b) for(ll i=a;i<=b;++i)

#define de(a) cout << #a << " = " << a << endl;
#define dd(a) cout << #a << " = " << a << "  ";

#define INF 0x3f3f3f3f
#define INFF 0x3f3f3f3f3f3f3f3f
#define mem(a,b)   memset(a,b,sizeof(a))
#define file freopen("in.txt","r",stdin)
#define jkl ios::sync_with_stdio(false);cin.tie(0);
/*********************************************/
const int N=1e4+5;
int main() {
	double a[N];
	int n;cin >> n;
	ff(i,1,n) {
		double aa;
		cin >> aa;
		a[i]=(double)1/aa+a[i-1];
	}
	cout << (double)1/a[n];
}

Measured memory_bytes: 256