#include<bits/stdc++.h>
#define N 101000
#define ll long long
#define GC getchar()
using namespace std;
ll re(){
	ll s=0,b=1; char c=GC;
	while(c>'9'||c<'0'){if(c=='-')b=-b; c=GC;}
	while(c<='9'&&c>='0'){s=(s<<1)+(s<<3)+(c^48); c=GC;}
	return s*b;
}
int n,a[N],cf[N],s,ans,s1;
int main(){
	n=re();
	for(int i=1; i<=n; ++i)a[i]=re();
	cf[1]=a[1]-a[n];
	for(int i=2; i<=n; ++i)cf[i]=a[i]-a[i-1];
	for(int i=1; i<=n; ++i)s=max(s,cf[i]);
	ans+=s;
	for(int i=1; i<=n; ++i)cf[i]-=s;
	bool fl=0;
	for(int i=1; i<=n; ++i){
		if((-cf[i])%n!=0){cout<<"-1"; return 0;}
		s1+=(-cf[i])/n; 
		if(cf[i]!=0)
		 if(fl)ans++;
		  else fl=1;
	} 
	if(s1!=s)cout<<"-1";
	 else if(ans==s){
	 	if(fl==0&&a[1]!=0)cout<<"-1";
		 else cout<<ans+(a[1]!=0);
	 } else cout<<ans;
}