/*
 * 143.cpp
 *
 *  Created on: 2017/08/26
 *      Author: pinebooks
 */

#include <cstdio>
#include <vector>

using namespace std;

int main() {
  int K, N, F;
  scanf("%d%d%d", &K, &N, &F);
  int sum = 0;
  for (int i = 0; i < F; ++i) {
    int a;
    scanf("%d", &a);
    sum += a;
  }
  if (K * N >= sum)
    printf("%d", K * N - sum);
  else
    printf("%d", -1);
  return 0;
}