#!/usr/bin/env python3
# -*- coding: utf-8-*-
if __name__ == '__main__':
    k, n, f = map(int, input().split(" "))
    a = list(map(int, input().split(" ")))
    ans = k*n - sum(a)
    if ans < 0:
        print("-1")
    else:
        print(ans)