#include "bits/stdc++.h"
using namespace std;
#define ll long long
#define endl '\n'
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep2(i,a,b) for(int i=int(a);i<int(b);i++)
#define pb emplace_back
#define all(x) (x).begin(),(x).end()
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
	ll n;
	cin>>n;
	vector<ll> a(n);
	rep(i,n)cin>>a[i];

	ll ans=-1;

	rep(i,n){
		rep(j,n){
			if(i!=j){
				ll tmp=a[i]*a[j];
				string s=to_string(tmp);
				bool f=true;
				rep2(k,1,s.size()){
					if((s[k]-'0')-(s[k-1]-'0')!=1)f=false;
				}
				if(f)ans=max(ans,tmp);
			}
		}
	}
	cout<<ans<<endl;
}

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