#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define fi first
#define se second
#define sz(a) (int)(a.size())
#define all(a) a.begin(),a.end()
#define lb lower_bound
#define ub upper_bound
#define owo ios_base::sync_with_stdio(0);cin.tie(0);
#define debug(...) fprintf(stderr, __VA_ARGS__),fflush(stderr)
#define time__(d) for(long blockTime = 0; (blockTime == 0 ? (blockTime=clock()) != 0 : false);\
debug("%s time : %.4fs\n", d, (double)(clock() - blockTime) / CLOCKS_PER_SEC))
typedef long long int ll;
typedef long double ld;
typedef pair<ll,ll> PII;
typedef pair<int,int> pii;
typedef vector<vector<int>> vii;
typedef vector<vector<ll>> VII;
ll gcd(ll a,ll b){if(!b)return a;else return gcd(b,a%b);}
const int MAXN = 1e6+5;
int a[MAXN],n,k;
ll dp[555][555],p[MAXN];
int main()
{
string s;
cin>>n>>k>>s;
int cur = 0,nxt = 0;
for(int i=0;i<2*n;i++){
if(s[i] == 'A')cur++;
else a[++nxt] = cur;
}
for(int i=1;i<=n;i++)p[i] = p[i-1]+a[i];
for(int i=0;i<=n;i++){
for(int j=0;j<=k;j++){
dp[i][j] = 1e9;
}
}
dp[0][0] = 0;
for(int x=1;x<=k;x++){
int l = 0;
for(int i=1;i<=n;i++){
while(l+1<=i && a[l+1] <= i)l++;
for(int j=l;j>=0;j--){
dp[i][x] = min(dp[i][x],dp[j][x-1] + (l-j)*i - (p[l]-p[j]));
}
}
}
cout<<dp[n][k];
}
