def pairss(n):
res = [];i = 1
while i < n:
for k in range(0,n,i*2):
for j in range(i):res.append((j+k,j+i+k))
i *= 2
return res
n = int(input());z = 1
while z * 2 < n:z *= 2
q = pairss(z);res = []
for a, b in q:res.append((a+1, b+1))
for a, b in q:res.append((n-a, n-b))
print(len(res))
for a, b in res:print(a,b)#include<bits/stdc++.h>
#define pb push_back
using namespace std;
typedef pair<int,int>pii;
const int maxn=1e5+10;
vector<pii>ans;
int n,ind,K;
inline void match(int l,int r)
{
if(l==r)return;int mid=(l+r)>>1;
match(l,mid),match(mid+1,r);
for(int i=l;i<=mid;i++) ans.pb({i,i-l+mid+1});
}
inline void solve()
{
cin>>n,ind=log2(n),K=(1<<ind);
match(1,K),match(n-K+1,n);
cout<<ans.size()<<'\n';
for(auto [l,r]:ans) cout<<l<<" "<<r<<'\n';
}
signed main(){ios::sync_with_stdio(false);cin.tie(NULL);return solve(),0;}805B - 3-palindrome | 805C - Find Amir |
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1729F - Kirei and the Linear Function | 25D - Roads not only in Berland |
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1040A - Palindrome Dance | 372A - Counting Kangaroos is Fun |
1396B - Stoned Game | 16A - Flag |
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1418C - Mortal Kombat Tower | 1382B - Sequential Nim |
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1538E - Funny Substrings | 234A - Lefthanders and Righthanders |