#include<bits/stdc++.h>
using namespace std;
int main(){
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int delim = 0;
  for(int i = 0; i < n - 1; ++i){
    if(s[i] != s[i + 1]) delim++;
  }
  cout << n - 1 - max(0, (delim - 2 * k)) << endl;
}