【构造】Codeforces Round #423 (Div. 1, rated, based on VK Cup Finals) B. High Load

#include<cstdio>
using namespace std;
int n,K,Ks[200010],x[200010],y[200010],ans,e;
int main(){
scanf("%d%d",&n,&K);
for(int i=1;i<=K;++i){
Ks[i]=1;
}
int now=2;
for(;;){
for(int i=1;i<=K;++i,++now){
if(now>n){
goto OUT;
}
x[++e]=Ks[i];
y[e]=now;
Ks[i]=now;
if(i==1){
++ans;
}
else if(i==2){
++ans;
}
}
}
OUT:
printf("%d\n",ans);
for(int i=1;i<=e;++i){
printf("%d %d\n",x[i],y[i]);
}
return 0;
}