#include<bits/stdc++.h>

 using namespace std;

 typedef long long ll;

 typedef unsigned int uint;

 typedef unsigned long long ull;

 typedef pair<int,int> PII;

 typedef pair<ll,ll> Pll;

 typedef vector<int> VI;

 typedef vector<PII> VII;

 //typedef pair<ll,ll>P;

 #define N  200010

 #define M  200010

 #define fi first

 #define se second

 #define MP make_pair

 #define pb push_back

 #define pi acos(-1)

 #define mem(a,b) memset(a,b,sizeof(a))

 #define rep(i,a,b) for(int i=(int)a;i<=(int)b;i++)

 #define per(i,a,b) for(int i=(int)a;i>=(int)b;i--)

 #define lowbit(x) x&(-x)

 #define Rand (rand()*(1<<16)+rand())

 #define id(x) ((x)<=B?(x):m-n/(x)+1)

 #define ls p<<1

 #define rs p<<1|1

 const ll MOD=1e9+,inv2=(MOD+)/;

       double eps=1e-;

       ll INF=1e15;

       int dx[]={-,,,};

       int dy[]={,,-,};

 struct node

 {

     int x,y,z,id;

 }a[N],b[N];

 bool cmp(node a,node b)

 {

     if(a.x!=b.x) return a.x<b.x;

     if(a.y!=b.y) return a.y<b.y;

     return a.z<b.z;

 }

 int read()

 {

    int v=,f=;

    char c=getchar();

    while(c<||<c) {if(c=='-') f=-; c=getchar();}

    while(<=c&&c<=) v=(v<<)+v+v+c-,c=getchar();

    return v*f;

 }

 int main()

 {

     int n=read();

     rep(i,,n)

     {

         a[i].x=read(),a[i].y=read(),a[i].z=read();

         a[i].id=i;

     }

     sort(a+,a+n+,cmp);

     int i,j,k,m=;

     for(i=;i<=n;)

     {

         j=i;

         while(j<=n&&a[i].x==a[j].x&&a[i].y==a[j].y) j++;

         for(k=i;k+<=j;k+=) printf("%d %d\n",a[k].id,a[k+].id);

         if(k==j-) b[++m]=a[k];

         i=j;

     }

     n=;

     for(i=;i<=m;)

     {

         j=i;

         while(j<=m&&b[i].x==b[j].x) j++;

         for(k=i;k+<=j;k+=) printf("%d %d\n",b[k].id,b[k+].id);

         if(k==j-) a[++n]=b[k];

         i=j;

     }

     for(i=;i<=n;i+=) printf("%d %d\n",a[i].id,a[i+].id);

     return ;

 }