#include<iostream>
#define REP(i,f) for (int i = 0; i < f; i++)
#define PRINT(s) cout << (s) << endl
using namespace std;
int main(){
	int k, n, f, a, sum = 0;
	cin >> k >> n >> f;
	REP(i, f){
		cin >> a;
		sum += a;
	}
	int ans = k * n - sum;
	(ans < 0) ? PRINT(-1) : PRINT(ans);
	return 0;
}