#include<bits/stdc++.h>
using namespace std;
// define
#define execute cerr << " Time: " << fixed << setprecision(6) << (1.0 * clock() / CLOCKS_PER_SEC) << "s\n";
#define ll long long
#define ii pair <int , int>
#define iii pair <int , ii>
#define se second
#define fi first
#define all(v) (v).begin() , (v).end()
#define Unique(v) sort(all(v)) , v.resize(unique(all(v)) - v.begin())
#define bit(x,i) (((x) >> (i)) & 1LL)
#define flip(x,i) ((x) ^ (1LL << (i)))
#define ms(d,x) memset(d , x , sizeof(d))
#define exist __exist
#define ends __ends
#define visit visited
#define left __left
#define right __right
#define prev __prev
#define next __next
#define sitingfake 1
#define orz 1
//constant
const long long mod = 1e9 + 7;
const long long linf = 4557430888798830399LL;
const long long nlinf = -4485090715960753727LL;
const int LOG = 20;
const int inf = 1061109567;
const int ninf = -1044266559;
const int dx[] = {0 , -1 , 0 , 1};
const int dy[] = {-1 , 0 , 1 , 0};
template<typename T> bool maximize(T &a, const T &b)
{
if(a < b) {a = b; return 1;}
return 0;
}
template<typename T> bool minimize(T &a, const T &b)
{
if(a > b) {a = b; return 1;}
return 0;
}
void Plus(ll & a ,ll b)
{
b %= mod;
a += b;
if(a < 0) a += mod;
a %= mod;
return;
}
void Mul(ll & a, ll b)
{
(a *= (b % mod)) %= mod;
return;
}
//code
const int maxn = 1e6 + 7;
int a[maxn] , b[maxn];
string s;
int n , k;
int timerA , timerB;
namespace sub3
{
ll dp[505][505];
ll pref[550];
ll sum(int l , int r)
{
return 1ll * ((r * (r + 1)) / 2) - 1ll * ((l * (l - 1)) / 2);
}
ll cost(int l , int r)
{
if(a[l] < b[l])
{
int pos = upper_bound(a + 1 , a + n + 1 , b[l]) - a;
return pref[r] - pref[pos - 1] - sum(b[l] , b[l] + (r - pos));
}
else
{
int pos = upper_bound(b + 1 , b + n + 1 , a[l]) - b - 1;
int len = pos - l + 1;
return pref[r] - pref[l - 1] - sum(a[l] - len , a[l] - len + (r - l));
}
}
void solve()
{
for(int i = 1; i <= n; i++)
{
pref[i] += pref[i - 1] + a[i];
}
ms(dp , 0x3f);
dp[0][0] = 0;
for(int i = 1; i <= k; i++)
{
for(int j = i; j <= n; j++)
{
for(int t = j; t >= 1; t--)
{
minimize(dp[i][j] , dp[i - 1][t - 1] + cost(t , j));
}
}
}
cout << dp[k][n];
}
}
void solve(void)
{
cin >> n >> k;
cin >> s;
for(int i = 0; i < s.size(); i++)
{
if(s[i] == 'A') a[++timerA] = i;
else b[++timerB] = i;
}
if(n <= 500) sub3 :: solve();
}
/**
3 1
BBBAAA
**/
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
#define task ""
if(fopen(task".inp","r"))
{
freopen(task".inp","r",stdin);
freopen(task".out","w",stdout);
}
int tc = 1;
// cin >> tc;
while(tc--) solve();
// execute;
}
