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)
    for i := 0; i < F; i++{
        fmt.Scan(&a[i])
    }
    
    var ans int
    ans = K * N - sum(a...)
    if ans < 0 {
        ans = -1
    }
    fmt.Print(ans)
}