go语言的通道疑惑

```go func sum(a []int, c chan int) { total := 0 for _, v := range a { total += v } c <- total } func main() { a := []int{7, 2, 8, -9, 4, 0} c := make(chan int) go sum(a[:len(a)/2], c) // 17 go sum(a[len(a)/2:], c) // -5 x, y := <-c, <-c fmt.Println(x, y, x+y) ``` 为什么这段代码，***变量x***输出的值是***-5***，***而不是17？***