first = input()

first_split = first.split()

K = int( first_split[0] )
N = int( first_split[1] )
F = int( first_split[2] )

A = list( map( int, input().split() ) )

sum = K * N

for i in range(F):

    sum = sum - A[i]

if sum < 0:

    print("-1")

else:

    print(sum)