package main

import "fmt"

func main() {
	var N, K, F, sum int
	fmt.Scan(&N, &K, &F)
	for i := 0; i < F; i++ {
		var v int
		fmt.Scan(&v)
		sum += v
	}
	ans := -1
	if N*K >= sum {
		ans = N*K - sum
	}
	fmt.Println(ans)
}