#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef __int128 lll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pld;
#define MAX 9223372036854775807LL
#define MIN -9223372036854775807LL
#define INF 0x3f3f3f3f3f3f3f3f
#define fi first
#define se second
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); cout << fixed; cout.precision(10);
#define sp << " "
#define en << "\n"
#define compress(v) sort(v.begin(), v.end()), v.erase(unique(v.begin(), v.end()), v.end())
ll n, K;
string s;
ll h[2000010];
ll x, y;
ll nu[2000010];
ll idx[2000010];
ll p = 1;
ll dp[2000010], yuk[2000010];
struct line
{
ll A, B;
ld S;
ll I;
};
ld gyo(ll A1, ll B1, ll A2, ll B2)
{
return (ld)(B2 - B1) / (ld)(A1 - A2);
}
struct CHT
{
vector<line> lin;
void init(void)
{
lin.clear();
}
void update(ll X, ll Y, ll I)
{
while(!lin.empty())
{
if(gyo(X, Y, lin.back().A, lin.back().B) < lin.back().S)
lin.pop_back();
else
break;
}
if(lin.empty())
{
lin.push_back({X, Y, -INF, I});
return;
}
lin.push_back({X, Y, gyo(X, Y, lin.back().A, lin.back().B), I});
}
pll query(ll X)
{
ll l = 0, r = (ll)lin.size() - 1;
while(l <= r)
{
ll mid = (l + r) >> 1;
if(lin[mid].S <= X)
l = mid + 1;
else
r = mid - 1;
}
return {lin[r].A * X + lin[r].B, lin[r].I};
}
}cht;
pll solve(ll lambda)
{
deque<pll> dq;
cht.init();
cht.update(0, n * n + 1, 0);
for(ll i = 1 ; i <= n ; i++)
{
dp[i] = nu[i] + lambda;
yuk[i] = 1;
while(!dq.empty() && dq.front().fi <= i)
{
ll num = dq.front().se;
cht.update(-num, dp[num] + num * (idx[num] - 1) - nu[idx[num] - 1], num);
dq.pop_front();
}
if(!dq.empty())
{
if(dp[i] > dp[dq.front().se] + lambda)
{
dp[i] = dp[dq.front().se] + lambda;
yuk[i] = yuk[dq.front().se] + 1;
}
}
pll gap = cht.query(i);
gap.fi += nu[i];
if(dp[i] > gap.fi + lambda)
{
dp[i] = gap.fi + lambda;
yuk[i] = yuk[gap.se] + 1;
}
while(!dq.empty() && dp[dq.front().se] > dp[i])
dq.pop_back();
dq.push_back({idx[i], i});
}
return {dp[n], yuk[n]};
}
int main(void)
{
fastio
cin >> n >> K;
cin >> s;
for(ll i = 0 ; i < n * 2 ; i++)
{
if(s[i] == 'A')
x++, h[x] = y;
else
y++;
}
for(ll i = 1 ; i <= n ; i++)
nu[i] = nu[i - 1] + h[i];
for(ll i = 1 ; i <= n ; i++)
{
while(p <= n && h[p] < i)
p++;
idx[i] = max(i + 1, p);
}
ll l = -n * n, r = n * n;
ll ans = -INF;
while(l <= r)
{
ll mid = (l + r) >> 1;
pll gap = solve(mid);
ans = max(ans, gap.fi - mid * K);
if(gap.se <= K)
r = mid - 1;
else
l = mid + 1;
}
cout << ans;
return 0;
}
