#include <stdio.h>

int eval(int , int * );

int main( int argc , char * argv[] ) {

    int i , j , num , ele[10000] , output[10000] , totalCase = 0;

    do {
        scanf("%d" , &num);
        if(num == -1)
            break;

        totalCase ++;

        for( j = 0 ; j < num ; j++ ) {
            scanf( "%d" , &ele[j] );
        }

        output[totalCase - 1] = eval(num , ele);


    } while (1);

    for( i = 0 ; i < totalCase ; i++ ) {
        printf("%d\n",output[i]);
    }
    return 0;
}

int eval(int num , int * ele) {

    int i , sum = 0 , average = 0;
    float avg = 0.0;
    int count = 0 , check = 0;
    for(i = 0 ; i < num ; i++) {
        sum += ele[i];
    }
    avg = (float) (sum / (float) num);

    average ++;

    if(avg - (int) avg == 0) {
        average = (int) avg;
        for(i = 0 ; i < num ; i++ ) {
            int diff = ele[i] - average;
            //printf(" Checking for element = %d , average = %d\n" , ele[i] , average);
            if(diff > 0) {
                check = check + diff;
                count += diff;
                //printf("Removing from element %d and check = %d\n" , diff , check);
            }
            else if( diff < 0) {

                check = check + diff;
                //count += (-1 * diff);
                //printf("moving into element %d and check = %d\n" , diff , check);
            }
        }
        if(check == 0)
            return count;
        else
            return -1; 
    }
    else {
        return -1;
    }


    return 0;
}