#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#define lll __int128_t
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
struct line
{
ll a,b,cnt,pl;
};
int n,k;
int arr[2000005];
pll dp[1000005];
int P[2000005];
ll sum[1000005];
vi zeros = {-1};
vi ones = {-1};
deque<line> hull;
void add_line(line l)
{
while(siz(hull) >= 2)
{
line l1 = hull.front();
hull.pop_front();
line l2 = hull.front();
pair<lll,lll> p1 = {l1.b-l2.b,l2.a-l1.a};
if(p1.ss < 0)
{
p1.ff *= -1;
p1.ss *= -1;
}
pair<lll,lll> p2 = {l1.b-l.b,l.a-l1.a};
if(p2.ss < 0)
{
p2.ff *= -1;
p2.ss *= -1;
}
if(p2.ff*p1.ss > p1.ff*p2.ss)
{
hull.push_front(l1);
break;
}
}
hull.push_front(l);
}
line get_best(ll x)
{
if(siz(hull) == 0) return (line){0,(ll)1e18,0,0};
while(siz(hull) >= 2)
{
line l1 = hull.back();
hull.pop_back();
if(l1.a*x+l1.b <= hull.back().a*x+hull.back().b)
{
hull.push_back(l1);
break;
}
}
return hull.back();
}
pll solve(ll c)
{
hull = {};
rep(i,n/2+3) dp[i] = {1e18,0};
dp[0] = {0,0};
vector<line> to_add;
int cur_add = 0;
rep(i,siz(ones))
{
while(cur_add < siz(to_add) && to_add[cur_add].pl <= i) add_line(to_add[cur_add++]);
line l = get_best(i);
dp[i] = min(dp[i],{l.a*i+l.b+sum[i]+c,l.cnt+1});
if(i == siz(ones)-1) break;
ll p = max(i+1,P[ones[i+1]]);
if(zeros[p] < ones[i+1])
{
dp[p] = min(dp[p],{dp[i].ff+c,dp[i].ss+1});
p++;
}
if(dp[i].ff < 1e18) to_add.pb({-i,dp[i].ff+p*i-i-sum[p-1],dp[i].ss,p});
}
return {dp[siz(ones)-1].ff-dp[siz(ones)-1].ss*c,dp[siz(ones)-1].ss};
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
cin >> n >> k;
string s;
cin >> s;
n *= 2;
rep(i,n) if(s[i] == 'B') arr[i] = 1;
int p = 0;
rep(i,n)
{
if(arr[i] == 0)
{
sum[siz(zeros)] = sum[siz(zeros)-1]+p;
zeros.pb(i);
}
else
{
p++;
ones.pb(i);
}
}
p = 0;
forall(it,ones)
{
while(p+1 < siz(zeros) && zeros[p+1] < it) p++;
P[it] = p;
}
ll l = 0;
ll r = 4e12;
ll ans = 0;
while(l <= r)
{
ll c = (l+r)/2;
pll ans2 = solve(c);
if(ans2.ss <= k)
{
ans = ans2.ff-c*(k-ans2.ss);
r = c-1;
}
else l = c+1;
}
cout << ans << "\n";
}
