process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', (line) => {
  lines.push(line);
});

reader.on('close', () => {
  var N = Number(lines[0]);
  var ary = lines[1].split(' ');
  var invAs = 0
  for (i = 0; i < N; i++){
    invAs += 1.0 / Number(ary[i])
  }
  console.log( 1.0 / invAs )
});
