#include <bits/stdc++.h>
using namespace std;

#define NDEBUG
#include <cassert>


typedef long long ll;
typedef long double Double;
typedef unsigned long long ull;
typedef pair<int,int> ii;
typedef pair<ll,ll> llll;
typedef pair<double,double> dd;

typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<ii> vii;
typedef vector<vector<ii>> vvii;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<llll> vllll;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<long double> vD;

#define sz(a)  int((a).size())
#define pb  push_back
#define eb  emplace_back
#define FOR(var,from,to) for(int var=(from);var<=(to);++var)
#define rep(var,n)  for(int var=0;var<(n);++var)
#define rep1(var,n)  for(int var=1;var<=(n);++var)
#define repC2(vari,varj,n)  for(int vari=0;vari<(n)-1;++vari)for(int varj=vari+1;varj<(n);++varj)
#define repC3(vari,varj,vark,n)  for(int vari=0;vari<(n)-2;++vari)for(int varj=vari+1;varj<(n)-1;++varj)for(int vark=varj+1;vark<(n);++vark)
#define ALL(c)  (c).begin(),(c).end()
#define RALL(c)  (c).rbegin(),(c).rend()
#define tr(i,c)  for(auto i=(c).begin(); i!=(c).end(); ++i)
#define mset(arr,val)  memset(arr,val,sizeof(arr))
#define mid(x,y) ((x)+((y)-(x))/2)
#define IN(x,a,b) ((a)<=(x)&&(x)<=(b))
#define cons make_pair
#define clamp(v,lo,hi) min(max(v,lo),hi)

template <typename T>
inline bool found(set<T> &s, T elem) { return s.find(elem) != s.end(); }
template <typename T>
inline bool found(unordered_set<T> &s, T elem) { return s.find(elem) != s.end(); }
template <typename T, typename U>
inline bool found(map<T,U> &s, U elem) { return s.find(elem) != s.end(); }
template <typename T, typename U>
inline bool found(unordered_map<T,U> &s, U elem) { return s.find(elem) != s.end(); }
template <typename T>
inline bool found(vector<T> &s, T elem) { return find(s.begin(), s.end(), elem) != s.end(); }
inline int found(string &s, string t) { return (int)s.find(t); }
inline int found(string &s, int c) { return (int)s.find(c); }

template<typename T1, typename T2> inline void amin(T1 & a, T2 const & b) { if (a>b) a=b; }
template<typename T1, typename T2> inline void amax(T1 & a, T2 const & b) { if (a<b) a=b; }
template<typename X, typename T> auto vectors(X x, T a) { return vector<T>(x, a); }
template<typename X, typename Y, typename Z, typename... Zs> auto vectors(X x, Y y, Z z, Zs... zs) { auto cont = vectors(y, z, zs...); return vector<decltype(cont)>(x, cont); }

inline ll square(ll x) { return x * x; }
inline ll gcd(ll a, ll b) { while(a) swap(a, b%=a); return b; }
template <typename T>
inline T mod(T a, T b) { return ((a % b) + b) % b; }

template <typename T>
int find_left(vector<T>& v, T elem) {
    return (int)(upper_bound(v.begin(), v.end(), elem) - v.begin()) - 1;
}
template <typename T>
int find_right(vector<T>& v, T elem) {
    return (int)(lower_bound(v.begin(), v.end(), elem) - v.begin());
}

const ll MOD=1000000007LL;

ll ADD(ll x, ll y) { return (x+y) % MOD; }
ll SUB(ll x, ll y) { return (x-y+MOD) % MOD; }
ll MUL(ll x, ll y) { return x*y % MOD; }
ll POW(ll x, ll e) { ll v=1; for(; e; x=MUL(x,x), e>>=1) if (e&1) v = MUL(v,x); return v; }
ll DIV(ll x, ll y) { /*assert(y%MOD!=0);*/ return MUL(x, POW(y, MOD-2)); }

vector<ii> rle(string& s) {
    vector<ii> work;
    char last = -1;
    int count = 0;
    for (char c : s) {
        if (c == last) {
            ++count;
        } else {
            if (count > 0) {
                work.emplace_back(last, count);
            }
            last = c; count = 1;
        }
    }
    if (count > 0) {
        work.emplace_back(last, count);
    }
    return work;
}

int solve(int N, int K, string& s) {
    vii r = rle(s); int R = r.size();
    int ans = 0;
    rep(i,R) ans += r[i].second - 1;
    int lo, hi;
    if (R % 2) {
        lo=1; hi=R-1;
    } else {
        lo=2; hi=R-1;
        ans += 1; --K;
    }
    while (lo < hi) {
        if (K == 0) break;
        ans += 2; --K;
        ++lo; --hi;
    }
    if (R % 2 == 0 && lo < hi) ++ans;
    return ans;
}

int main() {
    int N, K; cin >> N >> K;
    string s; cin >> s;
    cout << solve(N,K,s) << endl;
    return 0;
}


Measured memory_bytes: 1528