#include <algorithm>
#include <iostream>
#include <iomanip>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <cmath>
using namespace std;

#define FOR(i,s,e) for (int i = int(s); i < int(e); i++)
#define FORIT(i,c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define ISEQ(c) (c).begin(), (c).end()

int main(){
	int K,N,F;
	cin >> K >> N >> F;
	int mame = K*N;
	int eat = 0;
	FOR(i,0,F) {
		int tmp;
		cin >> tmp;
		eat += tmp;
	}

	int ans;
	if (eat > mame) ans = -1;
	else ans = mame -eat;

	cout << ans << endl;
	return 0;
}