package main

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

func main() {
	var k, n, f int
	fmt.Scan(&k, &n, &f)
	a := make([]int, f)
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	ans := k * n
	for i := range a {
		sc.Scan()
		a[i], _ = strconv.Atoi(sc.Text())
		ans -= a[i]
		if ans < 0 {
			fmt.Println(-1)
			return
		}
	}
	fmt.Println(ans)
}