/**
____ ____ ____ ____ ____ ____
||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 int maxn = 5100;
int n, k;
string s;
int dp[maxn][maxn], pos[2][maxn], pref[maxn];
/**
*/
void chmin(int &var, int val) /// careful for long long
{
var = min(var, val);
}
///int cost[maxn][maxn];
void precalc()
{
for (int j = 1; j <= n; j ++)
{
int tk = 0;
for (int fix = 1; fix <= n - j; fix ++)
{
if (pos[0][fix + j] > j * 2 + fix)
tk += pos[0][fix + j] - (j * 2 + fix);
///cost[j + 1][j + fix] = tk;
}
}
}
int cost(int left, int right)
{
int above = (left - 1) * 2;
int lf = left, rf = right;
while(lf <= rf)
{
int mf = (lf + rf) / 2;
if (pos[0][mf] > above + (mf - left + 1))
rf = mf - 1;
else
lf = mf + 1;
}
int sum = pref[right] - pref[lf - 1];
sum = sum - (right - lf + 1) * above;
int len = right - left + 1;
int sm = lf - left;
sum = sum - (len) * (len + 1) / 2;
sum = sum + (sm) * (sm + 1) / 2;
/**for (int i = lf; i <= right; i ++)
{
sum = sum + pos[0][i] - (above + (i - left + 1));
}*/
return sum;
}
void solve()
{
cin >> n >> k >> s;
s = "#" + s;
int cntA = 0, cntB = 0;
for (int 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 (int i = 0; i <= n; i ++)
for (int j = 0; j <= n; j ++)
dp[i][j] = 4 * n * n; /// careful for overflow
precalc();
int cr = 0;
for (int j = 1; j <= n; j ++)
{
cr = cr + pos[0][j] - j;
dp[j][1] = cr;
}
for (int i = 1; i <= n; i ++)
for (int j = 2; j <= k; j ++)
{
for (int fix = 1; fix <= i; fix ++)
{
if (dp[i - fix][j - 1] != 4 * n * n)
{
dp[i][j] = min(dp[i][j], dp[i - fix][j - 1] + cost(i - fix + 1, i));
}
}
}
int ans = 4 * n * n;
for (int i = 1; i <= k; i ++)
ans = min(ans, dp[n][i]);
cout << ans << endl;
}
int main()
{
///freopen("test.txt", "r", stdin);
solve();
return 0;
}
