#include <iostream>
#include <sstream>
#include <math.h>
#include <stack>
#include <set>
#include <algorithm>
#include <string>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <vector>
using namespace std;

#define INF (1 << 30)
#define INFLL (1LL << 60)

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