def main():
    K,N,F = map(int, input().split())
    A = sum(map(int, input().split()))

    if A > K*N:
        print(-1)
    else:
        print(K*N - A)

if __name__ == "__main__":  # {{{
    try:
        import test
        test.test()
    except:
        main()  # }}}