#include <iostream>
#include <algorithm>
#include <string>
using namespace std;

int main() {
	int k, n, f, sum=0, ans=0;
	int a[101];
	cin >> k >> n >> f;
	for(int i=0;i<f;i++){
		cin >> a[i];
		sum += a[i];
	}
	
	if(k*n >= sum){
		ans = k*n - sum;
	}else{
		ans = -1;
	}
	cout << ans << endl;
	
	return 0;
}