function Main(input) {
  lines = input.split('\n')
  N = lines[0] * 1
  s = lines[1].split(" ").map(function(a) { return 1 / a * 1 })
  ans = s.reduce(function(i, a) {
      return i + a
  }, 0)
  console.log(1 / ans)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))