f = input()
s_f = f.split()
K = int(s_f[0])
N = int(s_f[1])
F = int(s_f[2])
g = input()
s_g = g.split()
a = K * N
mame = 0
for v in s_g:
    mame += int(v)

if mame > a:
    print(-1)
else:
    b = a - mame
    print(b)