#include<stdio.h>

int main(){
	int k,n,f,i,m;
	scanf("%d %d %d",&k,&n,&f);
	int a[f];
	for (i=0;i<f;i++){
		scanf("%*c%d",&a[i]);
	}
	m = k*n;
	for (i=0;i<f;i++){
		m -= a[i];
	}
	printf("%d",(m<0)?-1:m);
	return 0;
}