function Main(input){
  var inputs = input.split("\n");
  var array = inputs[1].split(" ");
var N = inputs[0]-1;
  var sum = 0;
var i;
  for(i = 0;i <= N;i++){
    sum = sum + new Number(array[i]);
 }
  var plo;
  for(j = 0;j <= N;j++){
    if(plo == undefined){
      plo =new Number(array[j]);
    }
    plo = plo*new Number(array[j]);
  }
  var mom = 0;
  for(k = 0;k <= N;k++){
    mom = mom + plo/new Number(array[k]);
  }
  console.log(plo/mom);
  }
  Main(require('fs').readFileSync('/dev/stdin','utf8'));