K, N, F = input().split()
K, N, F = int(K), int(N), int(F)
A = input().split()
A = [int(i) for i in A]

beannum = K*N
for i in range(len(A)):
    beannum -= A[i]
    if(beannum < 0):
        break

if beannum >= 0:
    print(beannum)
else:
    print(-1)