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

int main(void){

	int K;    //一袋に入っている豆の粒数
	int N;    //袋数
	int F;    //家族の人数
	int A[101] = {0};    //年齢
	int A_COUNT = 0;
	int F_COUNT;
	int KN_TMP=0;

	scanf("%d %d %d", &K, &N, &F);

	F_COUNT = F;

	while (F_COUNT != 0) {    //家族の人数分年齢を取得
		scanf("%d", &A[A_COUNT]);
		A_COUNT++;
		F_COUNT--;
	}

	A_COUNT = A_COUNT -1;

	while (A_COUNT != 0 ) {
		KN_TMP += A[A_COUNT];
		A_COUNT--;
	}

	KN_TMP += A[A_COUNT];

	if ( (K * N) - KN_TMP < 0) {
		printf("%d\n", -1);
	}
	else {
		printf("%d\n", (K * N ) - KN_TMP);
	}

	return 0;
}