package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
)

func solve(in io.Reader, out, err io.Writer) {
	pack, num, family := 0, 0, 0
	fmt.Fscan(in, &pack, &num, &family)
	remain := pack * num
	for temp, i := 0, 0; i < family; i++ {
		fmt.Fscan(in, &temp)
		remain -= temp
	}
	if remain < -1 {
		remain = -1
	}

	fmt.Fprintln(out, remain)
}

func main() {
	br := bufio.NewReaderSize(os.Stdin, int(1e4))
	solve(br, os.Stdout, os.Stderr)
}