请将下述JavaScript代码转成Flow类型注解版本
```
function calculateAverage(numbers) {
  const sum = numbers.reduce((acc, num) => acc + num, 0);
  return sum / numbers.length;
}
```
user
```javascript
function calculateAverage(numbers: Array<number>): number {
  const sum: number = numbers.reduce((acc: number, num: number) => acc + num, 0);
  return sum / numbers.length;
}
```
assistant