def main():
    K ,N ,F = map(int,input().split(" "))
    A = [int(i) for i in input().split(" ")]
    age = 0

    for a in A:
        age = age + a

    ans = (K * N) - age
    if ans < 0:
        print(-1)
    else:
        print(ans)

if __name__=="__main__":
    main()