largest prime

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
typedef tree<ll,null_type,less_equal<ll>,rb_tree_tag,tree_order_statistics_node_update>ordered_set;
#define suni ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define endl "\n"
#define mn int main()
#define frac() cout.unsetf(ios::floatfield);cout.precision(6);cout.setf(ios::fixed,ios::floatfield);



const int mx=1e7+123;
bitset<mx>isprime;
vector<int>prime;
void primegen(int n)
{
    n+=10;
    for(int i=3;i<=n;i+=2) isprime[i]=1;
    for(int i=3;i*i<=n;i+=2){
            if(isprime[i]){
        for(int j=i*i;j<=n;j+=(i*2))
            isprime[j]=0;
      }
    } //O(n)
    isprime[2]=1;
    prime.push_back(2);
    for(int i=3;i<=n;i+=2){
        if (isprime[i]) prime.push_back(i);
    }
}

vector<long long >factorization(long long n)
{
    vector<long long >ret;
    for(auto p : prime){
        if(1LL*p*p>n) break;
        if(n%p==0){
            while(n%p==0){
                ret.push_back(p);
                n/=p;
            }
        }
    }
    if(n>1) ret.push_back(n);
    return ret;
}
mn
{
    suni;
    primegen(1e7);
    long long n;
    while(cin>>n)
    {
        if(n==0) break;
        vector<ll>ans=factorization(abs(n));
        if(ans.size()== 1) cout<<-1<<endl;
        else cout<<ans[ans.size()-1]<<endl;
    }

}