function main(arg) {
	arg = arg.trim().split('\n');
  	var a = arg[1].split(' ').map(str => parseInt(str, 10)),
        n = arg[0], out = 0;
  	//逆数の総和
    for (var i = 0; i < n; i++) {
      out += 1.0 / a[i];
    }
  	//逆数
    out = 1.0 / out
  console.log(out)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));