#include <iostream>
using namespace std;

int main() {
	// your code goes here
	int k,n,f,sum=0,ans=0;
	cin >> k >>n >>f;
	sum = k*n;
	for(int i = 0;i < f;i++){
		int num;
		cin >> num;
		ans+=num;
	}
	if(sum >= ans){
		cout <<sum - ans << endl;
	}
	else{
		cout <<"-1"<< endl;
	}
	return 0;
}