l = input().split()
for i in range(len(l)):
    i = int(i)
    l[i] = int(l[i])
k = l[0]
n = l[1]
f = l[2]

a = input().split()

for i in range(len(a)):
    i = int(i)
    a[i] = int(a[i])

mame = k*n

for i in range(f):
    mame -= a[i]

if(mame < 0):
    print(-1)
else:
    print(mame)