#include <stdio.h>
int main(){
int beans;
int bag;
int peoples;
int numbers;
int count;
count = 0;
numbers = 0;
scanf("%d", &beans);
scanf("%d", &bag);
scanf("%d", &peoples);
for(int n=1; n<=peoples; n++){
 scanf("%d", &numbers);
 count = count+numbers;
}
if (beans*bag-count>=0){
    printf("%d",beans*bag-count);
}
if (beans*bag-count<0){
    printf("-1");
}
return 0;
}