#include <stdio.h>

typedef struct {
  int b_nums;
  int s_nums;
  int total;
} beansInfo;

typedef struct {
  int fam_num;
  int age;
  int age_total;
} familyInfo;

void beans_sum(beansInfo *p);
void age_sum(familyInfo *p);
void print_result(beansInfo *dp, familyInfo *fp);

int main(void)
{
  beansInfo beans;
  familyInfo family;

  scanf("%d %d %d", &beans.b_nums, &beans.s_nums, &family.fam_num);

  beans_sum(&beans);
  age_sum(&family);
  print_result(&beans, &family);

  return 0;
}

void beans_sum(beansInfo *beans)
{
  beans->total = beans->b_nums * beans->s_nums;
}

void age_sum(familyInfo *family)
{
  family->age_total = 0;

  for (int i = 0; i < family->fam_num; ++i) {
    scanf("%d", &family->age);
    family->age_total += family->age;
  }
}

void print_result(beansInfo *beans, familyInfo *family)
{
  int result = beans->total - family->age_total;
  printf("%d\n", (result >= 0 ? result : -1));
}