const proto = {
  multiply ( a ) {
    this.n *= a;
    return this;
  },
  add ( a ) {
    this.n =+ a;
    return this;
  },
  divide ( a ) {
    this.n /= a;
    return this;
  },
  subtract ( a ) {
    this.n -= a;
    return this;
  },
  value () {
    return this.n;
  }
};

function given(n) {
  let instance = Object.create( proto );
  instance.n = n;
  return instance;
}