#include <bits/stdc++.h>
#define int long long 
using namespace std;
mt19937 rnd(chrono::system_clock().now().time_since_epoch().count());
int gen(int l,int r) {return rnd()%(r-l+1)+l;}
const int INF=2e5+5;
int n,a[INF],b[INF],cc[INF],ba2[INF],pp[INF],vis[INF];
int ksm(int x,int y,int Mod) {
	int ba=x%Mod,ans=1;
	while (y) {
		if (y&1) ans=(ans*ba)%Mod;
		ba=(ba*ba)%Mod;y>>=1;
	}
	return ans;
}
signed main()
{
	ios::sync_with_stdio(false);
	cin>>n;
	for (int i=0;i<n;i++) cin>>a[i]; 
	int res=1e18,fl=0;;
	for (int i=0;i<n;i++) 
		if (i*a[1]%n!=a[i]) fl=1;
	if (!fl) res=min(res,a[1]);
	for (int i=0;i<n;i++) b[i]=a[i]-i,b[i]%=n,b[i]+=n,b[i]%=n;
	for (int i=0;i<n;i++) b[i+n]=b[i];
	
	for (int y=1;y<n;y++) vis[y]=1;
	for (int T=1;T<=200;T++) {
		int ba=233,sum=0;
		for (int i=0;i<n;i++) pp[i]=gen(1,1e9);
		for (int i=0;i<n;i++) sum=(sum*ba+pp[i])%n;
		for (int i=0;i<n+n;i++) cc[i]=(cc[i-1]*ba+pp[b[i]])%n;
		ba2[0]=1;
		for (int i=1;i<=n+n;i++) ba2[i]=ba2[i-1]*ba%n;
		int Mod=n;
		for (int y=1;y<n;y++) {
			if (b[1+y]*sum%Mod!=((cc[n-1+y]-cc[0+y-1]*ba2[n])%Mod+Mod)%Mod) vis[y]=0;
		}
	}	
	for (int y=1;y<n;y++)
		if (vis[y]) res=min(res,b[1+y]+2);
	if (res>1e9) cout<<"-1\n";
	else cout<<res<<"\n";
	return 0;
}