// yukicoder: No.143 豆
// 2019.4.11 bal4u

#include <stdio.h>
#include <stdlib.h>

#if 0
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif
int in()
{
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int main()
{
	int i, K, N, F, s;

	K = in(), N = in(), F = in();
	s = 0; for (i = 0; i < F; i++) s += in();
	if (s > K*N) puts("-1");
	else printf("%d\n", K*N - s);
	return 0;
}