#include <stdio.h>

#define NMAX 10033
int n,m;
int v[NMAX];

int main()
{
  int i,tot;
  
  while (scanf("%d",&n)==1) {
    m=0;
    for(i=0;i<n;++i) {
      scanf("%d", &v[i]);
      m+=v[i];
    }
    if (m%n!=0) {
      printf("-1\n");
      continue;
    }
    m=m/n;
    tot=i=0;
    while (v[i]<=m && i<n) {
      tot+=m-v[i];
      i++;
    }
    printf("%d\n",tot+1);
  }
  return 0;
}