#include <stdio.h>

int main(void)
{
  int K, N, F, i,
      total, year_old = 0, rest;

  scanf("%d%d%d", &K, &N, &F);
  int family[F];
  for(i = 0; i < F; i++)  scanf("%d", &family[i]);
  total = K * N;
  for(i = 0; i < F; i++) year_old += family[i];
  if((rest = (total - year_old)) >= 0){
    printf("%d\n", rest);
  }else{
    printf("%d\n", -1);
  }

  return 0;
}