#include <bits/stdc++.h>

using namespace std;

#define int int64_t
#define F(i, n) for(int i = 0; i < n; ++ i)

int n, T, cnt = 0, lst_dom;
string str;
vector<int> v;

int check(int m) {
    string s = str;
    int ans2 = INT_MAX;
    int ans = 0, rem = cnt, tec = cnt - m;
    F(i, n) {
        if(!rem) break;
        if(s[i] == 'D') {
            if(!tec) {
                int pos = max(v[m - 1], lst_dom);
                ans2 = ans + (pos - i) * 2;
                for(int j = i + 1; j <= v[m - 1]; ++ j) {
                    if(s[j] == 'M') {
                        ++ tec;
                        s[j] = '.';
                    }
                    if(s[j] == 'D') {
                        -- tec;
                        s[j] = '.';
                        -- rem;
                    }
                    ans += 2;
                }
            }
            -- tec;
            -- rem;
        }
        if(str[i] == 'M') ++ tec;
        if(!rem) break;
        ++ ans;
    }
    return min(ans, ans2);
}

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin >> n >> T >> str;
    str = '.' + str; ++ n;
    F(i, n) {
        if(str[i] == 'M') v.push_back(i);
        if(str[i] == 'D') {
            ++ cnt;
            lst_dom = i;
        }
    }
    int l = 0, r = v.size() + 1;
    while(l < r) {
        int m = (l + r) / 2;
        if(check(m) <= T) {
            l = m + 1;
        } else r = m;
    }
    if(!l) {
        cout << -1;
        return 0;
    }
    cout << cnt - l + 1;
}