#include <bits/stdc++.h>

using namespace std;

typedef pair<int,int> P;
typedef pair<int,pair<int,int>> PP;
typedef long long ll;

const double EPS = 1e-8;
const int INF = 1e9;
const int MOD = 1e9+7;

int dy[] = {0,1,0,-1};
int dx[] = {1,0,-1,0};

int main(void) {
	int k,n,f;
	cin >> k >> n >> f;

	int ans = k*n;
	for(int i=0;i<f;i++){
		int a;
		cin >> a;
		ans -= a;
	}
	if(ans < 0)cout << -1 << endl;
	else cout << ans << endl;
	return 0;
}