#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define Would
#define you
#define please

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	int K, N, F;
	cin >> K >> N >> F;
	int KN = K * N;

	rep(i, F) {
		int a;
		cin >> a;
		KN -= a;
	}

	if (KN < 0) co(-1);
	else co(KN);



	Would you please return 0;
}