package main
import "fmt"

var k,n,f int

func main(){
    fmt.Scanf("%d%d%d",&k,&n,&f)
    s := k*n

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

    if s < -1 {
        s = -1
    }
    fmt.Println(s)
}