#include <iostream>
#include <stdio.h>
#include <math.h>
#include <vector>
#include <algorithm>
using namespace std;

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