#include "stdio.h"

int main(void){
    int k,n,f,i =0;
    int a[100];
    int sum =0;
    scanf("%d%d%d",&k,&n,&f);
    for(i=0;i<f;i++){
        scanf("%d",&a[i]);
        sum += a[i];
    }
    if((k*n)>= sum){ 
        printf("%d\n",((k*n)-sum)); 
     }else{ 
        printf("%d\n",-1);
     } 
     return 0;                    
}