import strutils
import sequtils

var
    knf = split(readLine(stdin), ' ').mapIt(int, it.parseInt)
    a_seq = split(readLine(stdin), ' ').mapIt(int, it.parseInt)
    sum_a = 0
    result : int

for a in a_seq:
    sum_a += a

result = -sum_a + knf[0] * knf[1]

if result < 0:
    echo(-1)
else:
    echo($result)