package main
import "fmt"

var k,n,f int

func main(){
    fmt.Scan(&k)
    fmt.Scan(&n)
    fmt.Scan(&f)
    s := k*n

    for i := 0; i < f; i++ {
        var a int
        fmt.Scan(&a)
        s -= a
    }

    if s < -1 {
        s = -1
    }
    fmt.Printf("%v\n", s)
}