#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define eb emplace_back
mt19937 rnd(1);
#define mod 998244353
#define ll long long
#define inf 0x3f3f3f3f
#define INF 0x3f3f3f3f3f3f3f3f
inline int read()
{
	char ch=getchar(); int nega=1; while(!isdigit(ch)) {if(ch=='-') nega=-1; ch=getchar();}
	int ans=0; while(isdigit(ch)) {ans=ans*10+ch-48;ch=getchar();}
	if(nega==-1) return -ans;
	return ans;
}
void print(vector<int> x){for(int i=0;i<(int)x.size();i++) printf("%d%c",x[i]," \n"[i==(int)x.size()-1]);}
#define N 600005
char s[2][N];
int n,k;
struct Treap
{
	int ch[N][2],val[N],siz[N],sum[N],key[N];
	void pushup(int u)
	{
		siz[u]=siz[ch[u][0]]+siz[ch[u][1]]+1;
		sum[u]=sum[ch[u][0]]+sum[ch[u][1]]+val[u];
	}
	int merge(int u,int v)
	{
		if(!u||!v) return u|v;
		if(key[u]<key[v])
		{
			ch[u][1]=merge(ch[u][1],v);
			pushup(u); return u;
		}
		else
		{
			ch[v][0]=merge(u,ch[v][0]);
			pushup(v); return v;
		}
	}
	pair<int,int> split(int u,int size)
	{
		if(!u) return {0,0};
		assert(siz[u]>=size);
		// printf("%d %d %d - %d %d\n",u,ch[u][0],ch[u][1],siz[u],size);
		if(siz[ch[u][0]]>=size)
		{
			auto [A,B]=split(ch[u][0],size);
			ch[u][0]=B;
			pushup(u);
			// printf("* %d %d\n",A,u);
			return {A,u};
		}
		else
		{
			auto [A,B]=split(ch[u][1],size-1-siz[ch[u][0]]);
			ch[u][1]=A;
			pushup(u);
			return {u,B};
		}
	}
	int find0(int rt) // return index from left
	{
		int cur=rt;
		if(sum[cur]==siz[cur]) return -1;
		int prew=0;
		while(1)
		{
			if(ch[cur][0]&&siz[ch[cur][0]]!=sum[ch[cur][0]]) cur=ch[cur][0];
			else if(val[cur]==0) return prew+siz[ch[cur][0]]+1;
			else prew+=siz[ch[cur][0]]+1,cur=ch[cur][1];
		}
	}
	int find0(int rt,int suf) // return index from left
	{
		auto [L,R]=split(rt,suf);
		int tw=find0(R);
		if(tw!=-1)
		{
			merge(L,R);
			return tw+suf;
		}
		else
		{
			merge(L,R);
			return find0(rt);
		}
	}
	int find1(int rt)
	{
		int cur=rt;
		if(sum[cur]==0) return -1;
		int prew=0;
		while(1)
		{
			if(ch[cur][0]&&0!=sum[ch[cur][0]]) cur=ch[cur][0];
			else if(val[cur]==1) return prew+siz[ch[cur][0]]+1;
			else prew+=siz[ch[cur][0]]+1,cur=ch[cur][1];
		}
	}
	int find1(int rt,int suf)
	{
		auto [L,R]=split(rt,suf);
		int tw=find1(R);
		if(tw!=-1)
		{
			merge(L,R);
			return tw+suf;
		}
		else
		{
			merge(L,R);
			return find1(rt);
		}
	}
	int qval(int rt,int rk)
	{
		int cur=rt;
		while(1)
		{
			if(rk==siz[ch[cur][0]]+1) return val[cur];
			else if(rk<=siz[ch[cur][0]]) cur=ch[cur][0];
			else rk-=siz[ch[cur][0]]+1,cur=ch[cur][1];
		}
	}
	void print(int rt)
	{
		if(!rt) return ;
		print(ch[rt][0]);
		printf("%d : %d %d - %d %d\n",rt,ch[rt][0],ch[rt][1],sum[rt],siz[rt]);
		print(ch[rt][1]);
	}
}tr;
int rta,rtb;
vector<pair<int,int>> ans;
/*
void doit(int x,int y)
{
	ans.emplace_back(x,y);
	if(x+k-1<=n&&y+k-1<=n)
	{
		int la,ma,ra;
		tie(la,ma)=tr.split(rta,x-1);
		tie(ma,ra)=tr.split(ma,k);
		int lb,mb,rb;
		tie(lb,mb)=tr.split(rtb,y-1);
		tie(mb,rb)=tr.split(mb,k);
		
		rta=tr.merge(tr.merge(la,mb),ra);
		rtb=tr.merge(tr.merge(lb,ma),rb);
	}
	else if(x+k-1<=n&&y+k-1>n)
	{
		int la,ma,ra;
		tie(la,ma)=tr.split(rta,x-1);
		tie(ma,ra)=tr.split(ma,k);
		int lb,mb,rb;
		tie(lb,mb)=tr.split(rtb,y+k-1-n);
		tie(mb,rb)=tr.split(mb,n-k);
		
		int tmp=tr.merge(rb,lb);
		rta=tr.merge(tr.merge(la,tmp),ra);
		auto [L,R]=tr.split(ma,n-y+1);
		rtb=tr.merge(tr.merge(R,mb),L);
	}
	else if(x+k-1>n&&y+k-1<=n)
	{
		int lb,mb,rb;
		tie(lb,mb)=tr.split(rtb,y-1);
		tie(mb,rb)=tr.split(mb,k);
		int la,ma,ra;
		tie(la,ma)=tr.split(rta,x+k-1-n);
		tie(ma,ra)=tr.split(ma,n-k);
		
		int tmp=tr.merge(ra,la);
		rtb=tr.merge(tr.merge(lb,tmp),rb);
		auto [L,R]=tr.split(mb,n-x+1);
		rta=tr.merge(tr.merge(R,ma),L);
	}
	else
	{
		int la,ma,ra;
		tie(la,ma)=tr.split(rta,x+k-1-n);
		tie(ma,ra)=tr.split(ma,n-k);
		int lb,mb,rb;
		tie(lb,mb)=tr.split(rtb,y+k-1-n);
		tie(mb,rb)=tr.split(mb,n-k);
		
		int ta=tr.merge(ra,la),tb=tr.merge(rb,lb);
		
		auto [L,R]=tr.split(tb,n-x+1);
		rta=tr.merge(tr.merge(R,ma),L);
		tie(L,R)=tr.split(ta,n-y+1);
		rtb=tr.merge(tr.merge(R,mb),L);
	}
}
*/
void doit(int x,int y)
{
	ans.emplace_back(x,y);
	{
		auto [L,R]=tr.split(rta,x-1);
		rta=tr.merge(R,L);
		tie(L,R)=tr.split(rtb,y-1);
		rtb=tr.merge(R,L);
	}
	auto [la,ra]=tr.split(rta,k);
	auto [lb,rb]=tr.split(rtb,k);
	rta=tr.merge(lb,ra);
	rtb=tr.merge(la,rb);
	// tr.print(rta),tr.print(rtb);
	// exit(0);
	{
		auto [L,R]=tr.split(rta,n-x+1);
		rta=tr.merge(R,L);
		tie(L,R)=tr.split(rtb,n-y+1);
		rtb=tr.merge(R,L);
	}
}
int norm(int x)
{
	while(x>n) x-=n;
	while(x<1) x+=n;
	return x;
}
signed main()
{
	cin>>n>>k;
	scanf("%s%s",s[0]+1,s[1]+1);
	for(int i=1;i<=n;i++)
	{
		if(s[0][i]=='B') tr.val[i]=0;
		else tr.val[i]=1;
		if(s[1][i]=='B') tr.val[i+n]=0;
		else tr.val[i+n]=1;
		tr.key[i]=rnd(),tr.key[i+n]=rnd();
		tr.pushup(i),tr.pushup(i+n);
	}
	rta=1,rtb=n+1;
	for(int i=2;i<=n;i++) rta=tr.merge(rta,i);
	for(int i=2;i<=n;i++) rtb=tr.merge(rtb,i+n);
	// return 0;
	// tr.print(rta),tr.print(rtb);
	int lsta=0,lstb=0;
	for(int i=1;i<=n-k;i++)
	{
		int ida=tr.find1(rta,lsta);
		int idb=tr.find0(rtb,lstb);
		lsta=ida,lstb=idb;
		if(ida==-1) break;
		// printf("%d %d\n",ida,idb);
		// if(i==3) return 0;
		doit(ida,idb);
	}
	// tr.print(rta),tr.print(rtb);
	// return 0;
	{
		int sum1=0;
		for(int i=1;i<=n;i++) sum1+=tr.qval(rtb,i);
		if(sum1-(n-k)<(k+1)/2)
		{
			int ida=tr.find1(rta,lsta);
			int idb=tr.find0(rtb,lstb);
			lsta=ida,lstb=idb;
			doit(ida,idb);
		}
	}
	while(1)
	{
		int ida=tr.find1(rta);
		int idb=tr.find0(rtb);
		if(ida==-1) break;
		doit(norm(ida+1),norm(idb-k+1));
		doit(ida,norm(idb-k+1));
	}
	cout<<ans.size()<<endl;
	for(auto [x,y]:ans) printf("%d %d\n",x,y);
	// for(int i=1;i<=n;i++) printf("%d%c",tr.qval(rta,i)," \n"[i==n]);
	// for(int i=1;i<=n;i++) printf("%d%c",tr.qval(rtb,i)," \n"[i==n]);
	return 0;
}