package main

import (
	"fmt"
)

func scan() (K, N, F int, A []int) {
	fmt.Scan(&K, &N, &F)
	for i := 0; i < F; i++ {
		var a int
		fmt.Scan(&a)
		A = append(A, a)
	}
	return
}

func main() {
	K, N, _, A := scan()
	// K, N, _, A := 10, 10, 3, []int{10, 20, 40}
	sum := K * N
	for _, a := range A {
		sum -= a
	}
	if sum < 0 {
		fmt.Print(-1)
	} else {
		fmt.Print(sum)
	}
}