first = input().split()
second = input().split()

f_int = [int(s) for s in first]
s_int = [int(s) for s in second]

total_beans = f_int[0] * f_int[1]
total_age = sum(s_int)

if(total_beans - total_age >= 0):print(total_beans - total_age)
else:print (-1)