#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<'\n'
#define dout(a) cout<<a<<' '<<#a<<'\n'
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<'\n'
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
//ifstream cin(".in");
//ofstream cout(".out");
in2(N,k);
string see; cin>>see;
vi v;
f0r(i, 2*N){
if(see[i] == 'A')v.pb(1);
else v.pb(-1);
}
int bad = -1;
int cur = 0;
f0r(i, 2 * N){
cur += v[i];
if(cur < 0){
bad = i; break;
}
}
int phaseone = 0;
while(bad != -1){
int nxt;
for(int i = bad + 1; i < 2*N; i++){
if(v[i] == 1){
nxt = i;
break;
}
}
for(int i = nxt; i>bad; i--){
phaseone++;
swap(v[i], v[i-1]);
}
int cur = 0;
bad = -1;
f0r(i, 2 * N){
cur += v[i];
if(cur < 0){
bad = i; break;
}
}
// dout(phaseone);
}
string se = "";
f0r(i, 2 * N){
if(v[i] == 1)se += 'A';
else se += 'B';
}
vi ass;
f0r(i, 2 * N){
if(se[i] == 'A')ass.pb(i);
}
// out(se);
// vout(v);
// out(phaseone);
vi perm;
f0r(i, N){
perm.pb(-1);
}
f0r(i, N){
perm.pb(1);
}
set<vi>s;
do{
int cur = 0;
bool ok = 1;
f0r(i, N*2){
cur = cur + perm[i];
if(cur < 0)ok = 0;
}
if(ok){
s.insert(perm);
}
}while(next_permutation(perm.begin(), perm.end()));
int anss = 4e18;
for(auto v : s){
vi as; vi bs;
f0r(i, 2 * N){
if(v[i] == 1)as.pb(i);
else bs.pb(i);
}
int ans = 0;
int fb = bs[0];
f0r(i, N){
if(as[i] > fb){
fb = bs[i];
ans++;
}
}
ans++;
if(ans <= k){
int cur = 0;
f0r(i, N){
cur += abs(ass[i] - as[i]);
}
anss = min(anss, cur);
}
}
out(anss + phaseone);
}
