/**
____ ____ ____ ____ ____ ____
||l |||e |||i |||n |||a |||d ||
||__|||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|/__\|
**/
#include<bits/stdc++.h>
#define endl '\n'
using namespace std;
typedef long long ll;
void speed()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
}
const ll maxn = 5100;
ll n, k;
string s;
ll dp[maxn][maxn], pos[2][maxn], pref[maxn];
/**
*/
void chmin(ll &var, ll val) /// careful for long long
{
var = min(var, val);
}
ll costd[maxn][maxn];
void precalc()
{
for (ll j = 1; j <= n; j ++)
{
ll tk = 0;
for (ll fix = 1; fix <= n - j; fix ++)
{
if (pos[0][fix + j] > j * 2 + fix)
tk += pos[0][fix + j] - (j * 2 + fix);
costd[j + 1][j + fix] = tk;
}
}
}
ll cost(ll left, ll right)
{
ll above = (left - 1) * 2;
ll lf = left, rf = right;
while(lf <= rf)
{
ll mf = (lf + rf) / 2;
if (pos[0][mf] > above + (mf - left + 1))
rf = mf - 1;
else
lf = mf + 1;
}
ll sum = pref[right] - pref[lf - 1];
sum = sum - (right - lf + 1) * above;
ll len = right - left + 1;
ll sm = lf - left;
sum = sum - (len) * (len + 1) / 2;
sum = sum + (sm) * (sm + 1) / 2;
/**for (ll i = lf; i <= right; i ++)
{
sum = sum + pos[0][i] - (above + (i - left + 1));
}*/
return sum;
}
void divide(ll row, ll left, ll right, ll optl, ll optr)
{
if (left > right)
return;
ll mid = (left + right) / 2;
ll opt = -1;
for (ll j = optl; j <= min(optr, mid); j ++)
{
ll cur =dp[j - 1][row - 1] + costd[j][mid];
if (cur < dp[mid][row])
{
dp[mid][row] = cur;
opt = j;
}
}
divide(row, left, mid - 1, optl, opt);
divide(row, mid + 1, right, opt, optr);
}
void solve()
{
cin >> n >> k >> s;
s = "#" + s;
ll cntA = 0, cntB = 0;
for (ll i = 1; i <= 2 * n; i ++)
{
if (s[i] == 'A')
{
cntA ++;
pos[0][cntA] = i;
pref[cntA] = pref[cntA - 1] + i;
}
else
{
cntB ++;
pos[1][cntB] = i;
}
}
for (ll i = 0; i <= n; i ++)
for (ll j = 0; j <= n; j ++)
dp[i][j] = 4 * n * n; /// careful for overflow
precalc();
ll cr = 0;
for (ll j = 1; j <= n; j ++)
{
cr = cr + pos[0][j] - j;
dp[j][1] = cr;
}
for (ll j = 2; j <= k; j ++)
{
divide(j, j, n, j, n);
}
cout << dp[n][k] << endl;
}
int main()
{
speed();
///freopen("test.txt", "r", stdin);
solve();
return 0;
}
