#include <bits/stdc++.h>
#define int long long 
using namespace std;
const int INF=2e5+5;
int n,a[INF],b[INF];
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<min(100ll,n);y++) {
		int fl=0;
		for (int i=0;i<n;i++)
			if (b[1+y]*i%n!=b[i+y]) fl=1;
		if (!fl) res=min(res,b[1+y]+2);
	}
	if (res>1e9) cout<<"-1\n";
	else cout<<res<<"\n";
	return 0;
}