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#include<stdio.h>
#include<cstring>
#include<algorithm>
#include<string.h>
#include<iostream>
#include<cmath>
#include<map>
#include<queue>
#include<numeric>
#include<set>
#pragma comment(linker, "/STACK:10240000000,10240000000")
using namespace std;
#define mem(x,y) memset(x,y,sizeof(x))
#define inf 0x3f3f3f3f
#define debug puts("-----")
#define maxn 50000+4
#define uLL unsigned long long
#define LL long long
int next[600000+6];
int b[5000+3];
int pre[5000+3];
int main()
{
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
int n;
mem(b,0);
mem(pre,-1);
scanf("%d",&n);
for(int i=0;i<2*n;i++)
{
int t;
scanf("%d",&t);
b[t]++;
next[i]=pre[t];
pre[t]=i;
}
for(int i=0;i<=5000;i++)
if(b[i]%2)
{
puts("-1");
return 0;
}
for(int i=0;i<=5000;i++)
{
for(int j=pre[i],cnt=0;~j;j=next[j],cnt++)
{
printf("%d",j+1);
printf("%c",cnt%2?'\n':' ');
}
}
}
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