1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
#include "bits/stdc++.h" // Tomasz Nowak
using namespace std;     // XIII LO Szczecin
using LL = long long;    // Poland
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
template<class T> int size(T &&x) {
	return int(x.size());
}
template<class A, class B> ostream& operator<<(ostream &out, const pair<A, B> &p) {
	return out << '(' << p.first << ", " << p.second << ')';
}
template<class T> auto operator<<(ostream &out, T &&x) -> decltype(x.begin(), out) {
	out << '{';
	for(auto it = x.begin(); it != x.end(); ++it)
		out << *it << (it == prev(x.end()) ? "" : ", ");
	return out << '}';
}
void dump() {}
template<class T, class... Args> void dump(T &&x, Args... args) {
	cerr << x << ";  ";
	dump(args...);
}
#ifdef DEBUG
  struct Nl{~Nl(){cerr << '\n';}};
# define debug(x...) cerr << (strcmp(#x, "") ? #x ":  " : ""), dump(x), Nl(), cerr << ""
#else
# define debug(...) 0 && cerr
#endif
mt19937_64 rng(0);
int rd(int l, int r) {
	return uniform_int_distribution<int>(l, r)(rng);
}
// end of templates

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	int k;
	cin >> k;

	vector<array<int, 2>> edges;
	auto new_vertex = [&] {
		int v = size(edges);
		edges.emplace_back(array<int, 2>{-1, -1});
		return v;
	};

	int start = new_vertex();
	int end = new_vertex();

	auto double_path = [&] {
		int s = start;
		edges[s][0] = new_vertex();
		start = new_vertex();
		edges[s][1] = start;
		edges[edges[s][0]][0] = start;
	};

	auto add1 = [&] {
		edges[start][0] = end;
		int s = new_vertex();
		edges[start][1] = s;
		start = s;
	};

	while(k) {
		if(k & 1)
			add1();
		double_path();
		k /= 2;
	}
	debug(edges);

	int e = new_vertex();
	edges[end][0] = e;

	cout << size(edges) << '\n';
	for(auto [e1, e2] : edges) {
		if(e1 >= 0)
			++e1;
		if(e2 >= 0)
			++e2;
		cout << e1 << ' ' << e2 << '\n';
	}
}