// Slower C++ solution:
#include <bits/stdc++.h>





//#define gc getchar unlocked

//#ifndef ONLINE JUDGE

#define gc getchar

//#endif // ONLINE JUDGE



//#define pc putchar_unlocked

//#ifndef ONLINE JUDGE

#define pc putchar

//#endif // ONLINE JUDGE



#define fRead           freopen("input.txt","r",stdin)

#define fWrite          freopen ("output.txt","w",stdout)



#define eps             1e-8

#define inf             0x3f3f3f3f

#define INF             2e18

#define LL              long long

#define ULL             unsigned long long

#define PI              acos(-1.0)

#define pb              push_back

#define mk              make_pair

#define pii             pair<int,int>

#define pLL             pair<LL,LL>

#define ff              first

#define ss              second

#define all(a)          a.begin(),a.end()

#define rall(a)         a.rbegin(),a.rend()

#define SQR(a)          ((a)*(a))

#define Unique(a)       sort(all(a)),a.erase(unique(all(a)),a.end())

#define min3(a,b,c)     min(a,min(b,c))

#define max3(a,b,c)     max(a,max(b,c))

#define min4(a,b,c,d)   min(min(a,b),min(c,d))

#define max4(a,b,c,d)   max(max(a,b),max(c,d))

#define max5(a,b,c,d,e) max(max3(a,b,c),max(d,e))

#define min5(a,b,c,d,e) min(min3(a,b,c),min(d,e))

#define Iterator(a)     __typeof__(a.begin())

#define rIterator(a)    __typeof__(a.rbegin())

#define FOR(a,it)       for(Iterator(a) it = a.begin();it != a.end(); it++)

#define rFOR(a,it)      for(rIterator(a) it = a.rbegin();it != a.rend(); it++)

#define FastRead        ios_base::sync_with_stdio(0);cin.tie(0)

#define CasePrint       pc('C'); pc('a'); pc('s'); pc('e'); pc(' '); write(qq++,false); pc(':'); pc(' ')



using namespace std;



const int MOD = 1e9 + 7;

int fx[] = {+1,-1,+0,+0,+1,+1,-1,-1,+0};

int fy[] = {+0,+0,+1,-1,+1,-1,+1,-1,+0};

template <typename T> inline T GCD (T a,T b ) {a = abs(a);b = abs(b); if(a < b) swap(a, b); while ( b ) { a = a % b; swap ( a, b ); } return a;}

template <typename T> inline T EGCD(T a,T b,T &x,T &y){if(a == 0) {x = 0;y = 1;return b;}T x1, y1;T d = EGCD(b % a, a, x1, y1);x = y1 - (b / a) * x1;y = x1;return d;}

template <typename T> inline T LCM(T x,T y){T tp = GCD(x,y);if( (x / tp) * 1. * y > 9e18) return 9e18;return (x / tp) * y;}

template <typename T> inline T BigMod(T A,T B,T M = MOD){T ret = 1;while(B){if(B & 1) ret = (ret * A) % M;A = (A * A) % M;B = B >> 1;}return ret;}

template <typename T> inline T InvMod (T A,T M = MOD){return BigMod(A,M-2,M);}

/*---------------------------fast I/O------------------------------------*/

#define scani2(a,b) scani(a) , scani(b)

#define scani3(a,b,c) scani(a), scani(b), scani(c)

#define scani4(a,b,c,d) scani(a), scani(b), scani(c), scani(d)

#define scani5(a,b,c,d,e) scani(a), scani(b), scani(c), scani(d) , scani(e)

template <typename T> T scani(T &n){n = 0;bool negative = false;char c = gc();while( c < '0' || c > '9'){if(c == '-') negative = true;c = gc();}while(c >= '0' && c <= '9'){n = n*10 + c-48;c = gc();}if(negative) n = ~(n-1);return n;}

template <typename T> void write(T n,int type = true){if(n<0) {pc('-');n = -n;}if(!n) {pc('0');if(type==32) pc(' '); else if(type) pc('\n'); return;}char buff[22];int len = 0;while(n) buff[len++] = n%10+48,n/=10;for(int i=len-1;i>=0;i--) pc(buff[i]);if(type==32) pc(' '); else if(type) pc('\n');}

int scans(char *a){int i=0;char c = 0;while(c < 33) c = gc();while(c > 33){a[i++] = c;c = gc();}a[i] = 0;return i;}

/*********************************************** End of template *********************************************/

const int N = 1000 + 10;



int n;

LL in[N];



bool ck(string s)

{

    for(int i=1;i<s.size();i++) if(s[i] != s[i - 1] + 1) return false;

    return true;

}



int main()

{

    LL mx = -1;

    scani(n);

    for(int i=0;i<n;i++) cin >> in[i];

    for(int i=0;i<n;i++){

        for(int j=i+1;j<n;j++){

            LL m = in[i] * in[j];

            stringstream input ;

            input << m;

            string s = input.str();

            if(ck(s)){

                mx = max(mx, m);

            }

        }

    }

    write(mx);

//    cout << mx << endl;

    return 0;

}

// Faster C++ solution for the same problem (measured speedup: 5.346380269870694x):


#include <algorithm>

#include <bitset>

#include <cassert>

#include <cctype>

#include <climits>

#include <cmath>

#include <complex>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <ctime>

#include <functional>

#include <iomanip>

#include <list>

#include <map>

#include <numeric>

#include <queue>

#include <set>

#include <stack>

#include <string>

#include <vector>

using namespace std;



typedef long long ll;

const int N = 1005;

const int M = 1000000007;



int nums[N];



bool check(long long num){

    int last_num=-1;

    while(num>0){

        int now = num%10;

        num/=10;



        if(last_num==-1){

            last_num = now;

        }else{

            if(now!=last_num-1){

                return false;

            }

            last_num = now;

        }



    }

    return true;

}



int main() {

    int n;

    scanf("%d",&n);

    for(int i=0;i<n;i++){

        scanf("%d",&nums[i]);

    }

    long long result = -1;

    for(int i=0;i<n;i++){

        for(int j=i+1;j<n;j++){

            long long tmp = nums[i]*nums[j];

            if(check(tmp)){

                if(tmp>result)

                    result = tmp;

            }

        }

    }

    printf("%lld\n", result);

}