list1 = input().split()
K=list1[0]
N=list1[1]
F=list1[2]
all_mame=int(K)*int(N)
list2=input().split()
need_mame=0

for i in range(int(F)):
    need_mame=need_mame+int(list2[i])

nokori_mame=all_mame-need_mame

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