```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()

	var t int
	fmt.Fscan(in, &t)
	for ; t > 0; t-- {
		var n int
		fmt.Fscan(in, &n)
		a := make([]int, n)
		twos := 0
		for i := 0; i < n; i++ {
			fmt.Fscan(in, &a[i])
			if a[i] == 2 {
				twos++
			}
		}

		if twos%2 != 0 {
			fmt.Fprintln(out, -1)
		} else {
			target := twos / 2
			curr := 0
			for k := 1; k < n; k++ {
				if a[k-1] == 2 {
					curr++
				}
				if curr == target {
					fmt.Fprintln(out, k)
					break
				}
			}
		}
	}
}
```