// Slower C++ solution:
#pragma comment(linker, "/stack:640000000")



#include <algorithm>

#include <bitset>

#include <cassert>

#include <cctype>

#include <climits>

#include <cmath>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <fstream>

#include <iostream>

#include <iomanip>

#include <iterator>

#include <list>

#include <map>

#include <numeric>

#include <queue>

#include <set>

#include <sstream>

#include <stack>

#include <string>

#include <utility>

#include <vector>

using namespace std;



const double EPS = 1e-9;

const int INF = 0x7f7f7f7f;

const double PI=acos(-1.0);



#define    READ(f) 	         freopen(f, "r", stdin)

#define    WRITE(f)   	     freopen(f, "w", stdout)

#define    MP(x, y) 	     make_pair(x, y)

#define    PB(x)             push_back(x)

#define    rep(i,n)          for(int i = 1 ; i<=(n) ; i++)

#define    repI(i,n)         for(int i = 0 ; i<(n) ; i++)

#define    FOR(i,L,R) 	     for (int i = L; i <= R; i++)

#define    ROF(i,L,R) 	     for (int i = L; i >= R; i--)

#define    FOREACH(i,t)      for (typeof(t.begin()) i=t.begin(); i!=t.end(); i++)

#define    ALL(p) 	         p.begin(),p.end()

#define    ALLR(p) 	         p.rbegin(),p.rend()

#define    SET(p) 	         memset(p, -1, sizeof(p))

#define    CLR(p)            memset(p, 0, sizeof(p))

#define    MEM(p, v)         memset(p, v, sizeof(p))

#define    getI(a) 	         scanf("%d", &a)

#define    getII(a,b) 	     scanf("%d%d", &a, &b)

#define    getIII(a,b,c)     scanf("%d%d%d", &a, &b, &c)

#define    getL(a)           scanf("%lld",&a)

#define    getLL(a,b)        scanf("%lld%lld",&a,&b)

#define    getLLL(a,b,c)     scanf("%lld%lld%lld",&a,&b,&c)

#define    getC(n)           scanf("%c",&n)

#define    getF(n)           scanf("%lf",&n)

#define    getS(n)           scanf("%s",n)

#define    bitCheck(N,in)    ((bool)(N&(1<<(in))))

#define    bitOff(N,in)      (N&(~(1<<(in))))

#define    bitOn(N,in)       (N|(1<<(in)))

#define    iseq(a,b)          (fabs(a-b)<EPS)

#define    ff 	 first

#define    ss 	 second

#define    ll	 long long

#define    ull 	 unsigned long long





typedef pair<int,int>pii;

typedef vector<pii>vpii;

typedef vector<int>vi;

typedef vector<vi>vii;





template< class T > inline T _abs(T n) { return ((n) < 0 ? -(n) : (n)); }

template< class T > inline T _max(T a, T b) { return (!((a)<(b))?(a):(b)); }

template< class T > inline T _min(T a, T b) { return (((a)<(b))?(a):(b)); }

template< class T > inline T _swap(T &a, T &b) { a=a^b;b=a^b;a=a^b;}

template< class T > inline T gcd(T a, T b) { return (b) == 0 ? (a) : gcd((b), ((a) % (b))); }

template< class T > inline T lcm(T a, T b) { return ((a) / gcd((a), (b)) * (b)); }

template <typename T> string NumberToString ( T Number ) { ostringstream ss; ss << Number; return ss.str(); }



#ifdef howcum

     #define debug(args...) {cerr<<"Debug: "; dbg,args; cerr<<endl;}

#else

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

#endif



struct debugger{

    template<typename T> debugger& operator , (const T& v){

        cerr<<v<<" ";

        return *this;

    }

}dbg;



int call(int a,int b)

{

    int res=a*b;

    int ans=res;

    int flag=1;

    int now=res%10;

    res/=10;

    while(res)

    {

        int rem=res%10;

        if(rem>=now || rem!=now-1)

        {

            flag=0;

            break;

        }

        now=rem;

        res/=10;

    }

    if(flag)

        return ans;

    else

        return -1;

}



int main() {

    #ifdef howcum

        READ("in.txt");

        //WRITE("out.txt");

    #endif // howcum



    int n;

    while(cin>>n)

    {

        int a[n+1];

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

        {

            cin>>a[i];

        }



        int maxy=-1;



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

        {

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

            {

                int res=call(a[i],a[j]);

                maxy=max(maxy,res);

            }

        }

        cout<< maxy<<endl;

    }



    return 0;

}

// Faster C++ solution for the same problem (measured speedup: 5.2416568958967105x):


#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);

}