#include <bits/stdc++.h>
using namespace std;
#define FOR(i,k,n) for(int i = (int)(k); i < (int)(n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) a.begin(), a.end()
#define MS(m,v) memset(m,v,sizeof(m))
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int, int> pii;
const int MOD = 1000000007;
const int INF = MOD + 1;
const ld EPS = 1e-12;
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }

/*--------------------template--------------------*/

bool check(ll n)
{
	string s = to_string(n);
	REP(i, s.size() - 1)
	{
		if (s[i + 1] - s[i] != 1) return false;
	}
	return true;
}

int main()
{
	cin.sync_with_stdio(false); cout << fixed << setprecision(10);
	int n;
	cin >> n;
	vi v(n);
	REP(i, n) cin >> v[i];
	int ans = -1;
	REP(i, n)REP(j, i)
	{
		int tmp = v[i] * v[j];
		if (check(tmp)) chmax(ans, tmp);
	}
	cout << ans << endl;
	return 0;
}