#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e3 + 12, MOD = (int)1e9 + 7;
int n, k, x[N], y[N], pref[ 2 * N], col[N], opt[N];
ll dp[N], c[N][N];
void prec() {
for(int i = 1; i <= n; i++) {
for(int j = i; j >= 1; j--) {
c[j][i] = c[j + 1][i];
if(y[j] <= x[i]) {
c[j][i] += pref[x[i]] - pref[y[j] - 1];;
}
}
}
for(int i = 1; i < n; i++) {
for(int j = 1; j < n; j++) {
assert(c[i][j + 1] + c[i + 1][j] >= c[i][j] + c[i + 1][j + 1]);
}
}
}
ll add[N * 4];
pair<ll, int> t[N * 4];
void inc(int v, int val) {
add[v] += val;
t[v].first += val;
}
void push(int v, int tl, int tr) {
if(add[v]) {
int tm = (tl + tr) >> 1;
inc(v + v, add[v]);
inc(v + v + 1, add[v]);
add[v] = 0;
}
}
void upd(int l, int r, int val, int v = 1, int tl = 1, int tr = n) {
if(l > r || tl > r || l > tr) return;
if(tl >= l && tr <= r) {
inc(v, val);
} else {
push(v, tl, tr);
int tm = (tl + tr) >> 1;
upd(l, r, val, v + v, tl, tm);
upd(l, r, val, v + v + 1, tm + 1, tr);
t[v] = min(t[v + v], t[v + v + 1]);
}
}
void upd1(int pos, ll val, int v = 1, int tl = 1, int tr = n) {
if(tl == tr) {
t[v].first += val;
t[v].second = tl;
} else {
push(v, tl, tr);
int tm = (tl + tr) >> 1;
if(pos <= tm) upd1(pos, val, v + v, tl, tm);
else upd1(pos, val, v + v + 1, tm + 1,tr);
t[v] = min(t[v + v], t[v + v + 1]);
}
}
const ll inf = 1e18;
pair<ll, ll> get(int l, int r, int v = 1, int tl = 1, int tr = n) {
if(l > r || tl > r || l > tr) return {inf, inf};
if(tl >= l && tr <= r) return t[v];
push(v, tl, tr);
int tm = (tl + tr) >> 1;
return min(get(l, r, v + v, tl, tm), get(l, r, v + v + 1, tm + 1, tr));
}
pair<ll, ll> calc1(ll pen) {
for(int i = 1; i <= n; i++) {
dp[i] = 1e18;
col[i] = n + 1;
for(int j = i; j >= 1; j--) {
if(dp[j - 1] + c[j][i] - pen <= dp[i]) {
dp[i] = dp[j - 1] + c[j][i] - pen;
col[i] = col[j - 1] + 1;
opt[i] = j;
}
}
assert(opt[i] >= opt[i - 1]);
// cout << dp[i] << '\n';
}
return {dp[n], col[n]};
}
pair<ll, ll> calc(ll pen) {
for(int i = 1; i <= n * 4; i++) {
add[i] = 0;
t[i] = {0, 0};
}
int it = 1;
for(int i = 1; i <= n; i++) {
dp[i] = 1e18;
col[i] = n + 1;
upd1(i, dp[i - 1]);
while(it <= n && y[it] < x[i]) {
upd(1, it, 1);
it++;
}
auto [val, j] = get(1, i);
dp[i] = val - pen;
col[i] = col[j - 1] + 1;
// cout << i << ' ' << dp[i] << ' ' << j << '\n';
}
return {dp[n], col[n]};
}
void test() {
cin >> n >> k;
int _x = 0, _y = 0;
for(int i = 1; i <= n + n; i++) {
char a;
cin >> a;
pref[i] = pref[i - 1];
if(a == 'A') {
x[++_x] = i;
pref[i]++;
} else {
y[++_y] = i;
}
}
prec();
// calc(0);
// calc1(0);
// return;
ll l = -1e12, r = 1, res;
while(r - l > 1) {
ll mid = (l + r) / 2;
auto [val, t] = calc1(mid);
// cout << mid << ' ' << val << ' ' << t << '\n';
if(t > k) {
r = mid;
} else {
l = mid;
res = val + k * 1ll * mid;
}
}
cout << res << '\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int t = 1;
// cin >> t;
while(t--)
test();
return 0;
}
