#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
using namespace std;
typedef long long ll;
typedef pair<int, int> P;

int A[1001];

int main()
{
	int K, N, F;
	cin >> K >> N >> F;

	ll mame = K*N;

	for (int i = 0; i < F; i++)
		cin >> A[i];

	for (int i = 0; i < F; i++)
		mame -= A[i];

	if (mame < 0)cout << -1 << endl;
	else cout << mame << endl;
	return	0;
}