# include <iostream>
# include <algorithm>
# include <vector>
# include <string>
# include <set>
# include <map>
# include <cmath>
# include <iomanip>
# include <functional>
# include <utility>
# include <stack>
# include <queue>
# include <list>
using namespace std;
using LL = long long;
constexpr int MOD = 1000000000 + 7;
//足し算してオーバーフローで死なないように確認しようね
constexpr int INF = 1000000000;
const double PI = acos(-1);

int main() {
	int k, n, f;
	cin >> k >> n >> f;
	int a[100000];
	int sum = 0;
	for (int i = 0; i < f; i++) {
		cin >> a[i];
		sum += a[i];
	}
	cout << (sum > k*n ? -1 : k*n - sum) << endl;
}