#include <stdio.h>

int main(void)
{
	int tubu = 0;
	int fukuro = 0;
	int family = 0;
	int count = 0;
	int i = 0;
	int sum = 0;
	int total[100];
	
	scanf("%d%d%d",&tubu,&fukuro,&family);
	
	for (count=0; count<family; count++){
		scanf("%d%",&total[count]);
	}
	
	for (i=0; i<family; i++) {
        sum += total[i];
    }
    
    if (tubu*fukuro-sum<0){
    	printf("%d", 0);
    }else{
    	printf("%d", tubu*fukuro-sum);
    }
    
	return 0;
}