package main

import (
	"fmt"
)

func main() {
	var k, n, f int
	fmt.Scanf("%d %d %d", &k, &n, &f)
	require := 0
	for i := 0; i < f; i++ {
		var x int
		fmt.Scanf("%d", &x)

		require += x
	}

	if require > k*n {
		fmt.Println(-1)
	} else {
		fmt.Println(k*n - require)
	}
}