# -*- coding: utf-8 -*-
# https://yukicoder.me/problems/361

def main():
    k, n, _ = input().split()
    years = sum(map(int, input().split()))
    mames = int(k) * int(n)

    print(mames - years if mames - years >= 0 else -1)

if __name__ == '__main__':
    main()