#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define X first
#define Y second
#define SZ(a) ((int)a.size())
#define ALL(v) v.begin(), v.end()
#define pb push_back
int lft[1000005], lst[1000005];
ll prv[1000005];
pll dp[1000005];
pll operator+(const pll &a, const ll &b) { return pll(a.X + b, a.Y + 1); }
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n, k;
string s;
cin >> n >> k >> s;
int cntA = 0, cntB = 0;
for (int i = 0; i < n + n; ++i) {
if (s[i] == 'B') ++cntB;
else lft[++cntA] = cntB;
}
for (int i = 1; i <= n; ++i)
prv[i] = prv[i - 1] + lft[i];
for (int i = 0; i <= n; ++i)
lst[lft[i]] = i;
for (int i = 1; i <= n; ++i)
lst[i] = max(lst[i], lst[i - 1]);
for (int i = 0; i <= n; ++i)
lst[i] = max(i, lst[i]);
auto line = [&](int q) {
return pll(-q, dp[q].X - prv[lst[q]] + (ll)q * lst[q]);
};
auto challenge = [&](int x, int y, int z) {
auto [ax, bx] = line(x);
auto [ay, by] = line(y);
auto [az, bz] = line(z);
return (by - bx) * (ax - az) >= (bz - bx) * (ax - ay);
};
auto get_val = [&](int q, int j) {
auto [a, b] = line(q);
return pll(a * j + b, dp[q].Y);
};
const ll INF = 2e12;
auto cal = [&](ll mid) {
fill(dp, dp + n + 1, pll(INF, INF));
dp[0] = pll(0, 0);
deque<int> dq;
for (int j = 1, p = 0; j <= n; ++j) {
while (lst[p] <= j && p < j) {
while (SZ(dq) >= 2 && challenge(dq[SZ(dq) - 2], dq.back(), p))
dq.pop_back();
dq.pb(p);
++p;
}
if (p < j)
dp[j] = min(dp[j], dp[p] + mid);
while (SZ(dq) >= 2 && get_val(dq[0], j) >= get_val(dq[1], j))
dq.pop_front();
if (!dq.empty())
dp[j] = min(dp[j], get_val(dq[0], j) + (mid + prv[j]));
}
return dp[n];
};
ll l = 0, r = (ll)n * n;
while (l < r) {
ll mid = (l + r) >> 1;
if (cal(mid).Y <= k) r = mid;
else l = mid + 1;
}
cout << cal(l).X - k * l << "\n";
}
