#include <iostream>	
#include <stdlib.h>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <string>
using namespace std;

int main()
{	
	int K,N,F;
	cin>>K >> N >>F;
	int count = N*K;
	int familyCount = 0;
	int *A = new int[F];
	for(int i = 0; i < F; i++){
		int num;
		cin>>num;
		familyCount += num;
	}
	if( familyCount > count){
		cout << -1 << endl;
	}else{
		cout << count - familyCount << endl;
	}

	return 0;
}