"use strict"
const Main = (input)=>{
  let xs = input.trim().split("\n");
  let N = xs[0]-0;
  let A = xs[1].split(" ").map(v=>parseInt(v,10));
  let ans = 0;
  for(let i = 0; i<N; i++){
  ans+= 1/A[i];
  }
  let ans1 = 1/ans; 
  console.log(ans1);

}
Main(require("fs").readFileSync("/dev/stdin","utf8"));