#include "bits/stdc++.h"
// Begin Header {{{
#define let             const auto
#define all(x)          (x).begin(), (x).end()
#define rep(i, n)       for (i64 i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, s, t)   for (i64 i = (s), i##_limit = (t); i <= i##_limit; ++i)
#define repr(i, s, t)   for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define var(Type, ...)  Type __VA_ARGS__; input(__VA_ARGS__)
#define lowerBound(...)                 lowerBound_(__VA_ARGS__)
#define upperBound(...)                 upperBound_(__VA_ARGS__)
#define lowerBound_(begin, end, ...)    (lower_bound((begin), (end), __VA_ARGS__) - (begin))
#define upperBound_(begin, end, ...)    (upper_bound((begin), (end), __VA_ARGS__) - (begin))

#ifdef DBG
#define trace(...) trace_g(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...)
#endif

using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template<class T, class U>inline bool chmax(T &a, const U &b){return b>a && (a=b, true);}
template<class T, class U>inline bool chmin(T &a, const U &b){return b<a && (a=b, true);}
inline i64  sigma(i64 n)            { return (n * (n + 1) >> 1); }
inline i64  updiv(i64 a, i64 b)     { return (a + b - 1) / b; }
inline i64  sqr(i64 n)              { return n * n; }
inline string to_string(char c)     { return string(1, c); }
inline bool   isRangeIn(i64 a, i64 low, i64 high) { return (low <= a && a <= high); }
constexpr int INF  = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template<class T>
vector<T> makeVec(size_t sz) { return vector<T>(sz); }

template<class T, class... Args>
auto makeVec(size_t sz, Args... args) {
    return vector<decltype(makeVec<T>(args...))>(sz, makeVec<T>(args...));
}

template<class T>
inline void input(T &x) { cin >> x; }

template<class Head, class... Tail>
inline void input(Head &head, Tail&... tail) { cin >> head; input(tail...); }

inline void print() { cout << "\n"; }

template<class Head, class... Tail>
inline void print(Head &&head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << ' ';
    print(forward<Tail>(tail)...);
}

template<class T>
ostream& operator<< (ostream &out, const vector<T> &vec) {
    static constexpr const char *delim[] = { " ", "" };
    for (const auto &e : vec) out << e << delim[&e == &vec.back()];
    return out;
}

template<class T>
ostream& operator<< (ostream &out, const vector<vector<T>> &mat) {
    static constexpr const char *tail[] = { "\n", "" };
    for (const auto &row : mat) out << row << tail[&row == &mat.back()];
    return out;
}

template <class T>
void trace_g(const char *s, T&& x) {
    clog << '{';
    while(*s != '\0') clog << *(s++);
    clog << ":" << setw(3) << x << '}' << endl;
}

template <class Head, class... Tail>
void trace_g(const char *s, Head&& head, Tail&&... tail) {
    clog << '{';
    while(*s != ',') clog << *(s++);
    clog << ":" << setw(3) << head << "}, ";
    for (++s; !isgraph(*s); ++s);
    trace_g(s, std::forward<Tail>(tail)...);
}
// }}} End Header

template<class Iterator>
auto groupBySameElementSubsequence(Iterator begin, Iterator end) { // {{{
    using value_type = typename iterator_traits<Iterator>::value_type;

    vector<pair<value_type, int>> res;
    int cnt = 1;

    Iterator prevBegin;
    for (prevBegin = begin++;  begin != end;  prevBegin = begin++) {
        if (*prevBegin == *begin) {
            ++cnt;
        } else {
            res.emplace_back(*prevBegin, cnt);
            cnt = 1;
        }
    }
    res.emplace_back(*prevBegin, cnt);

    return res;
} // }}}

signed main()
{
    ios::sync_with_stdio(false); cin.tie(nullptr);

    var(int, N, K);
    var(string, s);

    const auto x = groupBySameElementSubsequence(all(s));

    i64 ans = 0;
    i64 cnt = x.size() - 1;
    for (const auto &e : x) {
        ans += e.second - 1;
    }

    while(K > 0 && cnt > 0) {
        if (cnt > 0) { ++ans; --cnt; }
        if (cnt > 0) { ++ans; --cnt; }
        --K;
    }

    print(ans);

    return 0;
}


Measured memory_bytes: 1480