#include <bits/stdc++.h>

typedef long double ld;
typedef long long ll;
using namespace std;
int di[] = {1, 0, -1, -1, 0, 1, -1, 1};
int dj[] = {1, 1, 0, -1, -1, 0, 1, -1};
const ll oo = 1e18, MOD = 998244353;
const int N = 1005, M = 105;
const ld PI = acos(-1.0), EPS = 1e-9;

//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;


int n;
bool yes[N][N];
pair<int, int> a[N];
vector<pair<int, int>> ans;

bool cmp(pair<int, int> one, pair<int, int> two) {
    if (one.first < two.first && yes[one.second][two.second]) {
        ans.push_back({one.second, two.second});
        return 1;
    }
    return 0;
}

//#define endl '\n'
int main() {
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    //freopen("farm.in", "r", stdin);
    //memset(dp, -1, sizeof dp);
    cin >> n;
    int m = 1 << n;
    for (int i = 0; i < m; i++) cin >> a[i].first, a[i].second = i;
    for (int i = 0; i < m; i++) {
        for (int j = i + 1; j < m; j++) {
            int num = 0;
            for (int k = 0; k < n; k++) {
                int num1 = min(1, a[i].first & (1 << k));
                int num2 = min(1, a[j].first & (1 << k));
                if (num1 ^ num2) num++;
            }
            if (num == 1) yes[i][j] = yes[j][i] = 1;
        }
    }
    sort(a, a + m, cmp);
    cout << ans.size() << endl;
    for (auto x:ans) cout << x.first << " " << x.second << endl;
    return 0;
}