package main

import (
    "fmt"
)

func sum(nums ...int) int {
    //	fmt.Print(nums, " ")
    total := 0
    for _, num := range nums {
        total += num
    }
    return total
}

func main() {
    var K, N, F int
    fmt.Scan(&K, &N, &F)
    var a []int
    a = make([]int, F)
    var ans int
    for i := 0; i < F; i++{
//        fmt.Scan(&c)
//        a = append(a, c)
        fmt.Scanf("%d", &a[i])
    }
    //	fmt.Print(a)
    //	var sum_a int
    //	sum_a = sum(a...)
    //	fmt.Println(sum_a)
    //	fmt.Println(sum(a...))
    ans = K * N - sum(a...)
    if ans >= 0 {
        fmt.Print(ans)
    } else {
        fmt.Print("-1")
    }
}