#include<iostream>
#include<cstdio>
#define N 1000
#define inf 1e18
using namespace std;
int read()
{
	char c=0;
	int sum=0;
	while (c<'0'||c>'9') c=getchar();
	while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
	return sum;
}
int n,k,s[N+1];
long long F[N+1][N+1],dp[N+1][N+1];
char c[N+1];
int main()
{
	n=read(),k=read();
	for (int i=1;i<=(n<<1);++i)
	{
		cin>>c[i];
		if (c[i]=='A') s[i]=s[i-1]+1;
		else s[i]=s[i-1]-1;
	}
	for (int i=0;i<=k;++i)
		for (int j=0;j<=(n<<1);++j)
			dp[i][j]=inf;
	dp[0][0]=0;
	for (int i=0;i<=(n<<1);++i)
		for (int j=i+2;j<=(n<<1);j+=2)
		{
			for (int k=i+1;k<=j;++k) F[i][j]+=((max(min(k-i,j-k)-s[k],0)+1)>>1);
		}
	for (int i=1;i<=k;++i)
		for (int j=0;j<=(n<<1);++j)
			for (int t=j;t>=0;t-=2)
				dp[i][j]=min(dp[i][j],dp[i-1][t]+F[t][j]);
	printf("%lld\n",dp[k][n<<1]);
	return 0;
}