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