#include<bits/stdc++.h>
#define ll long long
#define pll pair<ll, ll>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define ld long double
using namespace std;
const ll inf=2e9, maxn=1e3;
ll pre[maxn], to[maxn];
ll n, k, a[maxn], dp[maxn][maxn];
ll cost(ll j, ll i)
{
if (to[j]>i) return 0;
return (pre[i]-i*j)-(pre[to[j]-1]-(to[j]-1)*j);
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> k;
string s; cin >> s;
ll x=0, y=0, crr=1;
for (ll i=0; i<n*2; i++)
{
if (s[i]=='A')
x++, pre[x]=pre[x-1]+y;
else y++;
}
for (ll i=1; i<=n; i++)
while (crr<=pre[i]-pre[i-1])
to[crr]=i, crr++;
while (crr<=n) to[crr]=n+1, crr++;
for (ll i=1; i<=n; i++)
if (to[i]<=i)
to[i]=i+1;
for (ll i=0; i<=k; i++)
for (ll j=0; j<=n; j++)
dp[i][j]=inf*inf;
dp[0][0]=0;
for (ll i=1; i<=k; i++)
for (ll j=1; j<=n; j++)
for (ll z=0; z<j; z++)
dp[i][j]=min(dp[i][j], dp[i-1][z]+cost(z, j));
cout << dp[k][n];
}
