#include<iostream>
using namespace std;
int main()
{
	int i,j,count,move;
	long long int sum;
	cin >> count;
	while(count!=-1){
		int a[count];
		for(i=1;i<=count;i++){
			cin >> a[i];
		}
		sum=0;
		move=0;
		for(i=1;i<=count;i++){
			sum = sum + a[i];
		}
		if(sum%count!=0){
			cout << -1;
		}else{
			sum = sum/count;
			for(i=1;i<=count;i++){
				if(a[i]<sum){
					move = move + sum-a[i];
				}
			}
			cout << move << endl;
		}
		cin >> count;
	}
	return 0;
}
 

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NQoxCjEKMQoxCjYKMgozCjQKLTE=

5
1
1
1
1
6
2
3
4
-1