#include <bits/stdc++.h>   using namespace std;   int n, t, c, d[555555], res = 0, ll = -1;   bool try_ans(int k) {     if (d[n - 1] > k) return false;     int last = 0, now = 0, res, l2 = -1;     vector<pair<int, int> > aa;     for (int i = 0; i < n; i++) {       if (d[i] > k) {         l2 = i;         if (!last) aa.push_back(make_pair(i, now));         last = 1;         now += 2;       } else if (last == 1)         last = 2, l2 = i;       else         last = 0;     }     if (ll > l2) l2 = ll;     res = now + l2 + 1;     for (int i = aa.size() - 1; i >= 0; i--)       if (2 * l2 + 1 - aa[i].first + aa[i].second < res)         res = 2 * l2 + 1 - aa[i].first + aa[i].second;     return res <= t;   }   int ans(int max, int min) {     if (max == min) return min;     int mid = (max + min) / 2;     bool a = try_ans(mid);     if (a)       return ans(mid, min);     else       return ans(max, mid + 1);   }   int main() {     cin >> n >> t;     string str;     cin >> str;     int now = 0;     for (int i = 0; i < n; i++) {       c = str[i];       if (c ==  H ) now++, ll = i;       if (c ==  S ) now--;       d[i] = now;       if (now > res) res = now;     }     if (ll + 1 > t)       cout << -1;     else {       cout << ans(res, 0);     }     return 0;   }   