package main

import (
	"fmt"
)

func resolve(K, N, F int, A []int) int {
	r := K * N
	for i := 0; i < F; i++ {
		r -= A[i]
		if r < 0 {
			return -1
		}
	}
	return r
}

func main() {
	var K, N, F int
	fmt.Scanf("%d %d %d\n", &K, &N, &F)
	A := make([]int, F)
	for i := 0; i < F; i++ {
		fmt.Scanf("%d", &A[i])
	}
	fmt.Scanf("\n")
	fmt.Println(resolve(K, N, F, A))
}