//#include <bits/stdc++.h>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<map>
#include<queue>
#include<stack>
#include<vector>
#include<deque>
#include<functional>
#include<string>
#include<iostream>
#include<cctype>
#include<set>
#include<climits>
#include<iomanip>
#include<cassert>
#include<sstream>

#define pb push_back
#define nl puts ("")
#define sp printf ( " " )
#define phl printf ( "hello\n" )
#define ff first
#define ss second
#define POPCOUNT __builtin_popcountll
#define RIGHTMOST __builtin_ctzll
#define LEFTMOST(x) (63-__builtin_clzll((x)))
#define MP make_pair
#define FOR(i,x,y) for(vlong i = (x) ; i <= (y) ; ++i)
#define ROF(i,x,y) for(vlong i = (y) ; i >= (x) ; --i)
#define CLR(x,y) memset(x,y,sizeof(x))
#define UNIQUE(V) (V).erase(unique((V).begin(),(V).end()),(V).end())
#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))
#define NUMDIGIT(x,y) (((vlong)(log10((x))/log10((y))))+1)
#define SQ(x) ((x)*(x))
#define ABS(x) ((x)<0?-(x):(x))
#define FABS(x) ((x)+eps<0?-(x):(x))
#define ALL(x) (x).begin(),(x).end()
#define LCM(x,y) (((x)/gcd((x),(y)))*(y))
#define SZ(x) ((vlong)(x).size())
#define NORM(x) if(x>=mod) x-=mod;if(x<0) x+=mod;
#define MOD(x,y) (((x)*(y))%mod)
#define ODD(x) (((x)&1)==0?(0):(1))
#define Set(N,cur) N=(N|(1LL<<cur))
#define Reset(N,cur) N=(N&(~(1LL<<cur)))
#define Check(N,cur) (!((N&(1LL<<cur))==0))
#define fast_cin ios_base::sync_with_stdio(false);cin.tie(NULL)

using namespace std;


#define LL long long
#define LLU long long unsigned int
typedef long long vlong;
typedef unsigned long long uvlong;
typedef pair < int, int > pii;
typedef pair < vlong, vlong > pll;
typedef vector<int> vi;
typedef vector<vlong> vl;
typedef vector<pll> vll;

#ifdef forthright48
     #include <ctime>
     clock_t tStart = clock();
     #define debug(args...) {dbg,args; cerr<<endl;}
     #define timeStamp debug ("Execution Time: ", (double)(clock() - tStart)/CLOCKS_PER_SEC)
     #define bug printf("%d\n",__LINE__);

#else
    #define debug(args...)  // Just strip off all debug tokens
    #define timeStamp
#endif

struct debugger{
    template<typename T> debugger& operator , (const T& v){
        cerr<<v<<" ";
        return *this;
    }
}dbg;

inline vlong gcd ( vlong a, vlong b ) {
    a = ABS ( a ); b = ABS ( b );
    while ( b ) { a = a % b; swap ( a, b ); } return a;
}

vlong ext_gcd ( vlong A, vlong B, vlong *X, vlong *Y ){
    vlong x2, y2, x1, y1, x, y, r2, r1, q, r;
    x2 = 1; y2 = 0;
    x1 = 0; y1 = 1;
    for (r2 = A, r1 = B; r1 != 0; r2 = r1, r1 = r, x2 = x1, y2 = y1, x1 = x, y1 = y ) {
        q = r2 / r1;
        r = r2 % r1;
        x = x2 - (q * x1);
        y = y2 - (q * y1);
    }
    *X = x2; *Y = y2;
    return r2;
}

inline vlong modInv ( vlong a, vlong m ) {
    vlong x, y;
    ext_gcd( a, m, &x, &y );
    x %= m;
    if ( x < 0 ) x += m;
    return x;
}

inline vlong power ( vlong a, vlong p ) {
    vlong res = 1, x = a;
    while ( p ) {
        if ( p & 1 ) res = ( res * x );
        x = ( x * x ); p >>= 1;
    }
    return res;
}

inline vlong bigmod ( vlong a, vlong p, vlong m ) {
    vlong res = 1 % m, x = a % m;
    while ( p ) {
        if ( p & 1 ) res = ( res * x ) % m;
        x = ( x * x ) % m; p >>= 1;
    }
    return res;
}


//int knightDir[8][2] = { {-2,1},{-1,2},{1,2},{2,1},{2,-1},{-1,-2},{1,-2},{-2,-1} };
//int dir4[4][2] = {{-1,0},{0,1},{1,0},{0,-1}};
//int dir8[8][2] = {{-1,0},{0,1},{1,0},{0,-1},{-1,-1},{1,1},{1,-1},{-1,1}};
const vlong inf = 2147383647;
const vlong mod = 1000000007;
const double pi = 2 * acos ( 0.0 );
const double eps = 1e-9;

///======================  TEMPLATE ENDS HERE  =====================///


/** WARNING WARNING WARNING **/
//  1. Simulate Sample Test Case Before Coding
//  2. Read Input & Output Format Before Coding
//  3. Check Corner Case Specially When N<3
//  4. Check Array Size, Mod Value & Long Long
//  5. Check DP1 Memoization Part & Call Parameters
//  6. Check Problem No When Submitting

#define Size 5005

int A[10005];
int N;
vector<int> L;

bool ok(int cur){
    L.clear();
    while(cur>0){
        L.pb(cur%10);
        cur /= 10;
    }
    if(L.size() == 1) return true;
    for(int i = 1;i<L.size();i++){
        if(L[i]+1 != L[i-1]) return false;
    }
    return true;
}

void solve(){
    int res = -1;
    for(int i = 0;i<N;i++){
        for(int j = i+1;j<N;j++){
            int cur = A[i]*A[j];
            if(ok(cur)){
                res = max(res,cur);
            }
        }
    }
    cout << res << endl;
}

int main () {
    #ifdef forthright48
    //freopen ( "Ainput.txt", "r", stdin );
    //freopen ( "output.txt", "w", stdout );
    #endif // forthright48

    scanf("%d",&N);
    for(int i = 0;i<N;i++){
        scanf("%d",&A[i]);
    }
    solve();
    return 0;
}

Measured memory_bytes: 3220