#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stack>
using namespace std;
int n,data[1005];
int cal(int x) {
    int tx=x;
    int pre=-1;
    while(tx) {
        if(pre!=-1&&tx%10-pre!=-1) return -1;
        pre = tx%10;
        tx/=10;
    }
    return x;
}
int main() {
    scanf("%d",&n);
    for(int i=0;i<n;i++) scanf("%d",&data[i]);
    int ans = -1;
    for(int i=0;i<n;i++)
    for(int j=i+1;j<n;j++) {
        ans = max(ans,cal(data[i]*data[j]));
    }
    printf("%d\n",ans);
}