K, N, F = list(map(int, input().split()))
A = input().split()
A = sorted(A, key=int)
been = K * N
f = 0

for f in range(F):
    if been >= int(A[f]):
        been = been - int(A[f])
        f += 1
    else:
        been = -1
        print(been)
        break
if been != -1:
    print(been)