try:
    import mydebug

    def dprint(*objects, sep='', end='\n'):
        print(*objects, sep, end)
except:
    def dprint(*objects, sep='', end='\n'): pass

k, n, f = [int(x) for x in input().split()]
a = [int(x) for x in input().split()]

total = k * n
family = sum(a)

if total >= family:
    print(total - family)
else:
    print("-1")