#include<stdio.h>

int main(void){
  unsigned K,N,F,A=0,num,tmp;
  scanf("%u%u%u",&K,&N,&F);
  while(F--){
    scanf("%u",&tmp);
    A+=tmp;
  }
  num=K*N;
  if(num-A>=0) printf("%u\n",num-A);
  else puts("-1");
  return 0;
}