#include <stdio.h>
#include <algorithm>
#include <deque>
using namespace std;
using ll = long long;

//143
int main() {
	int k, n, f, a, total = 0;
	scanf("%d %d %d", &k, &n, &f);
	for (int i = 0; i < f; i++) {
		scanf("%d", &a);
		total += a;
	}
	if (total <= k * n)
		printf("%d\n", k * n - total);
	else
		printf("-1\n");
}