#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using i128 = __int128;
using u128 = unsigned __int128;
template <typename T>
void chkmax(T &x, const T &y) {
  if (x < y) x = y;
}
template <typename T>
void chkmin(T &x, const T &y) {
  if (y < x) x = y;
}
struct vector2 {
  ll x, y;
  ll operator()(int k) const { return y - x * k; }
};
vector2 operator-(const vector2 &u, const vector2 &v) {
  return {u.x - v.x, u.y - v.y};
}
i128 operator*(const vector2 &u, const vector2 &v) {
  return (i128)u.x * v.y - (i128)u.y * v.x;
}
constexpr int MAXN = 1e6 + 10;
constexpr ll INFLL = 0x3f3f3f3f3f3f3f3f;
int n, k, val[MAXN], num2[MAXN], qu1[MAXN], hd1, tl1, qu2[MAXN], hd2, tl2;
vector2 qu3[MAXN];
ll sum1[MAXN], sum2[MAXN], ans = -1;
pair<ll, int> dp[MAXN];
bool check(ll coef) {
  hd1 = hd2 = 1;
  tl1 = tl2 = 0;
  for (int i = 1; i <= n; ++i) {
    while (hd2 <= tl2 && dp[i - 1] <= dp[qu2[tl2]]) --tl2;
    qu2[++tl2] = i - 1;
    for (int j = val[i - 1]; j < val[i]; ++j) {
      if (qu2[hd2] == j) ++hd2;
      vector2 now = {j, dp[j].first + (ll)j * num2[j] - sum2[j]};
      while (hd1 < tl1) {
        i128 tmp = (qu3[tl1] - qu3[tl1 - 1]) * (now - qu3[tl1]);
        if (tmp < 0 || (!tmp && dp[j].second <= dp[qu1[tl1]].second)) {
          --tl1;
        } else {
          break;
        }
      }
      qu1[++tl1] = j;
      qu3[tl1] = now;
    }
    while (hd1 < tl1 &&
           (qu3[hd1](i) > qu3[hd1 + 1](i) ||
            (qu3[hd1](i) == qu3[hd1 + 1](i) &&
             dp[qu1[hd1]].second >= dp[qu1[hd1 + 1]].second))) {
      ++hd1;
    }
    dp[i] = min<pair<ll, int>>({dp[qu2[hd2]].first + coef, dp[qu2[hd2]].second + 1},
                               {qu3[hd1](i) + sum1[i] + coef, dp[qu1[hd1]].second + 1});
  }
  if (dp[n].second > k) return false;
  chkmax(ans, dp[n].first - k * coef);
  return true;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> n >> k;
  for (int i = 1, x = 0, y = 0; i <= n * 2; ++i) {
    char ch;
    cin >> ch;
    if (ch == 'A') {
      val[++x] = y;
    } else {
      ++y;
    }
  }
  ll cst = 0;
  for (int i = 1; i <= n; ++i) {
    if (val[i] >= i) {
      cst += val[i] - i + 1;
      val[i] = i - 1;
    }
    sum1[i] = sum1[i - 1] + val[i];
    ++num2[val[i]];
    sum2[val[i]] += val[i];
  }
  for (int i = 1; i <= n; ++i) {
    num2[i] += num2[i - 1];
    sum2[i] += sum2[i - 1];
  }
  ll l = 0, r = 1e12;
  while (l <= r) {
    ll mid = (l + r) >> 1;
    if (check(mid)) {
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
  cout << ans + cst << "\n";
  return 0;
}
// g++ chorus.cpp -o chorus -Wall -Wextra -Wshadow -O2 -std=c++14 -fsanitize=address,undefined