/*  This Code is Submitted by wywcgs for Problem 1370 on 2005-03-23 at 15:30:03 */ 
#include <stdio.h>

int main()
{
    int total, average, candy[100000], n, step;
    int i;
    
    while(scanf("%d", &n) == 1){
        if(n == -1){
            return 0;
        }else{
            total = 0;
            for(i = 0; i < n; i++){
                scanf("%d", &candy[i]);
                total += candy[i];
            }
            if(total % n != 0){
                printf("-1\n");
            }else{
                average = total / n;
                step = 0;
                for(i = 0; i < n; i++){
                    if(candy[i] < average){
                        step += (average - candy[i]);
                    }
                }
                printf("%d\n", step);
            }
        }
    }
    
    return 0;
}