package main

import (
	"fmt"
)

func sum(nums ...int) int {
//	fmt.Print(nums, " ")
	total := 0
	for _, num := range nums {
		total += num
	}
	return total
}

func main() {
	var K, N, F int
	fmt.Scan(&K, &N, &F)
	var a []int
	for i := 0; i < F; i++{
		var c int
		fmt.Scan(&c)
		a = append(a, c)
	}
//	fmt.Print(a)
//	var sum_a int
//	sum_a = sum(a...)
//	fmt.Println(sum_a)
//	fmt.Println(sum(a...))
	var ans int
	ans = K * N - sum(a...)
	if ans >= 0 {
		fmt.Print(ans)
	} else {
		fmt.Print("-1")
	}
}