这题是逆向思维贪心……当时咋没想到……

本题启示：memset的效率与for一遍差不多，故千万memset导致超时……

#include<cstdio>
#include<cstring>
#include<iostream>
#include<cstdlib>
#include<cmath>
using namespace std;
#define MAXN (1000000+10)
int n,r,minchange=MAXN;
bool ans[MAXN]={0},res[MAXN];
bool flag=0;
int calc()
{
	int tot=0;
	for (int i=2;i<=n;i++) if (!res[i]^res[i-1]) tot++;
	return tot;
}
void relax()
{
/*	for (int i=1;i<=n;i++) cout<<res[i]<<' ';
	cout<<endl;
*/	int now=calc();
	if (!flag||now<minchange)
	{
		flag=1;minchange=now;
		memcpy(ans,res,sizeof(ans));
		return;
	}
	if (now>minchange) return;


	for (int i=2;i<=n;i++)
	{
		if (ans[i]<res[i]) return ;
		if (ans[i]>res[i])
		{
			memcpy(ans,res,sizeof(ans));
			return;
		}
	}
}
bool is_ok(int l,int r)
{
//	memset(res,0,sizeof(res));
	res[1]=1;
	for (int i=n;i>1;i--)
	{
		if (!l||!r) return 0;
		if (r<l) {l=l-r; res[i]=1;}
		else {r-=l; res[i]=0;}
	}
	if (l!=1||r!=1) return 0;
	relax();
	for (int i=2;i<=n;i++) res[i]=!res[i];
	relax();
}
int main()
{
	cin>>n>>r;
	for (int i=1;i<=r;i++)
	{
		flag=is_ok(i,r)|flag;
//1		cout<<i<<endl;
	}
	if (!flag) cout<<"IMPOSSIBLEn";
	else
	{
		cout<<minchange<<'n';
		for (int i=1;i<=n;i++)
		{
			if (ans[i]) printf("T");
			else printf("B");
		}
		printf("n");
	}
//	while (1);
	return 0;
}