code: func GetInt64(r Retriever, k Key) (int64, error) {
  	val, err := r.Get(k)
  	if IsErrNotFound(err) {
  		return 0, nil
  	}
  	if err != nil {
  		return 0, err
  	}
  	intVal, err := strconv.ParseInt(string(val), 10, 0)
  	if err != nil {
  		return intVal, errors.Trace(err)
  	}
  	return intVal, nil
  }
docs: // GetInt64 get int64 value which created by IncInt64 method.