#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define int long long
#define dd double
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define pii pair<int, int>
#define Mod 998244353
const int N=1e5+10;
int n,a[N];
int check(int d){
	int oth=(a[0]!=0);
	for(int i=0;i<n;i++)
	  if((a[i]+d)%n!=a[(i+1)%n])
		return INT_MAX;
	return d+oth;
}
void solve(){
	cin>>n;
	for(int i=0;i<n;i++)cin>>a[i];
	int x=a[0],y=a[1];
	if(x>=y)y+=n;int ans=check(y-x);
	if(ans==INT_MAX)printf("-1");
	  else printf("%d",ans);
}
signed main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	int _ = 1;
	while (_--)
		solve();
    return 0;
}