#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main(void){
int k,n,f,a[101];
scanf("%d %d %d",&k,&n,&f);
int sum=0;
for(int i=0;i<f;i++){
    scanf("%d",&a[i]);
    sum=sum+a[i];
}
if(k*n>=sum){printf("%d\n",k*n-sum);}
else{printf("-1\n");}

}