#include <bits/stdc++.h>

using namespace std;

#define INF 2000000000
#define MOD 1000000007
typedef long long ll;
typedef pair<int, int> P;


int main()
{
	int k,n,f;
	cin >>k>>n>>f;
	int mx = k*n;
	for (int i = 0; i < f; i++) {
		int tmp;
		cin >> tmp;
		mx -= tmp;
	}

	if (mx<0) {
		cout << -1 << "\n";
		return 0;
	}
	cout << mx << endl;

}