#include <iostream>
using namespace std;

int main(void){
//input
	int K,N,F;
	cin >> K >> N >> F;
	
	int a[F];
	for(int i=0;i<F;i++){
		cin >> a[i];
	}
//calc
	int age_sum=0;
	int beans = K * N;
	for(int i=0;i<F;i++){
		age_sum += a[i];
	}
//output
	if(beans >= age_sum){
		cout << beans - age_sum;
	}else{
		cout << "-1" ;
	}
}