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

int main(){
	int K,N,F;
	cin >> K >> N >> F;
	vector<int> A(F);
	int rem = K*N;
	for(int i=0; i<F; i++){
		cin >> A[i];
		rem -= A[i];
	}
	cout << (rem<0?-1:rem) << endl;
	return 0;
}