Problem Statement

Yuki picked up $N$ bags, each containing exactly $K$ beans.
By the way, on Setsubun there is a custom of eating as many beans as one's age.
Yuki's family has $F$ members, whose ages are $A_1,A_2,\ldots,A_F$ years.
Find how many beans will finally remain after each of them eats as many beans as their age.
However, if not everyone can eat as many beans as their age, output $\verb|-1|$.

Figure 1. Today's haul (February 3, 2015, Seigan-ji Temple)

Input

$K$ $N$ $F$
$A_1$ $A_2$ $\cdots$ $A_F$

$1 \leq K \leq 1000$
$1 \leq N \leq 1000$
$1 \leq F \leq 100$
$1 \leq A_i \leq 100$

Output

Output the number of remaining beans, or output $\verb|-1|$.

Sample

10 10 3
10 20 40

30