package main

import "fmt"

func main() {
	var k, n, f int
	fmt.Scanf("%v %v %v", &k, &n, &f)
	w := make([]int, f)
	for i := 0; i < f; i++ {
		_, _ = fmt.Scan(&w[i])
	}
	beans := k * n
	old := 0
	for _, v := range w {
		old = old + v
	}
	if beans-old < 0 {
		fmt.Println(-1)
	} else {
		fmt.Println(beans - old)
	}
}