#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<int, int> Pr;

int K, N, F;

int main()
{
	cin >> K >> N >> F;
	int sum = K * N;
	for (int i = 0; i < F; i++) {
		int A; cin >> A;
		sum -= A;
	}
	if (sum < 0)
		cout << -1 << endl;
	else
		cout << sum << endl;
	return 0;
}