```go
package main

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

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

	var q int
	fmt.Fscan(in, &q)
	for ; q > 0; q-- {
		var n int
		var k int64
		fmt.Fscan(in, &n, &k)
		var lo int64 = 1
		var hi int64 = 1<<62 - 1
		for i := 0; i < n; i++ {
			var a int64
			fmt.Fscan(in, &a)
			l := a - k
			if l < 1 {
				l = 1
			}
			r := a + k
			if l > lo {
				lo = l
			}
			if r < hi {
				hi = r
			}
		}
		if lo > hi {
			fmt.Fprintln(out, -1)
		} else {
			fmt.Fprintln(out, hi)
		}
	}
}
```