#include<stdio.h>

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