from collections import defaultdict
import sys
sys.stdin = open("input.txt", "r")
sys.stdout = open("output.txt", "w")
n = int(input())*2
a = [int(i) for i in input().split()]
c = defaultdict(list)
for i in range(n):
c[a[i]].append(i+1)
ans = []
for [k, v] in c.items():
l = len(v)
if l % 2:
print('-1')
exit()
else:
for i in range(0, l-1, 2):
ans.append(f'{v[i]} {v[i+1]}')
print('\n'.join(ans))
#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<vector>
#include<stack>
#include<set>
#include<map>
#include<queue>
#include <iomanip>
using namespace std;
#define ll long long
ll gcd(ll a, ll b) { return ((b == 0) ? a : gcd(b, a % b)); }
ll lcm(ll a, ll b) { return (b / gcd(a, b)) * a; }
void hhh() {
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr), cout.tie(nullptr);
}
const double EPS = 1e-7;
int main() {
hhh();
int N;
cin >> N;
vector<int>v(N*2);
map<int, vector<int>>ma;
for (int i = 0; i < 2*N; i++) {
cin >> v[i];
ma[v[i]].push_back(i + 1);
}
for (auto it : ma) {
if (it.second.size() % 2 != 0) {
cout << -1;
return 0;
}
}
for (auto it : ma) {
for (int i = 0; i < it.second.size()-1; i+=2) {
cout << it.second[i] << " " << it.second[i + 1] << "\n";
}
}
return 0;
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