const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('', (N) => {
  rl.question('', (nums) => {
    const arr = nums.split(' ');
    const divNums = arr.map((num) => 1/parseInt(num));
    const sum = divNums.reduce((accum, curVal) => accum + curVal);
    console.log(1/sum);
  });
});