#include<stdio.h>

int main()
{
	int n;
	while(1)
	{
		scanf("%d",&n);
		if(n==-1)
			break;
		int i;
		long long total=0;
		int *arr=new int[n];
		for(i=0;i<n;i++)
		{
			scanf("%d",&arr[i]);
			total+=arr[i];
		}
		if(total%n)
		{
			printf("-1\n");
			continue;
		}
		int avg=total/n,result=0;
		for(i=0;i<n;i++)
			if(arr[i]>avg)
				result+=(arr[i]-avg);
		printf("%d\n",result);
	}
	return 0;
}