INF

#include <bits/stdc++.h>
#define Roc ios_base::sync_with_stdio(false), cin.tie(NULL)
#define ll int
#define NMAX 500009
using namespace std;
ll fr[4],n,k,act[4],ans,neww[4];
char c;
ifstream f("inf.in");
ofstream g("inf.out");
deque <char> Q;
int code(char c)
{
    if(c == 'I')
        return 1;
    if(c == 'N')
        return 2;
    if(c == 'F')
        return 3;
}
int main()
{
    act[1] = 0;
    act[2] = 1;
    act[3] = 2;
    f >> n >> k;
    for(int i = 1; i <= k; ++i)
    {
        f >> c;
        Q.push_back(c);
        int cod = code(c);
        for(int j = 1; j <= 3; ++j)
        {
            act[j]++;
            if(act[j] > 3) act[j] -= 3;
            fr[j] += ( cod != act[j]  );
        }
    }
    ans = fr[1];
    for(int i = 2; i <= 3; ++i)
        ans = min(ans, fr[i]);
    for(int i = k + 1; i <= n; ++i)
    {
        f >> c;
        Q.push_back(c);
        int cod = code(c);
        int cod2 = code(Q.front());
        Q.pop_front();
        for(int j = 1; j <= 3; ++j)
        {
            int prec = j - 1;
            if(prec == 0)
                prec = 3;
            neww[j] = fr[prec];
            neww[j] += ( cod != act[j]  );
            neww[j] -= ( cod2 != prec  );
        }
        for(int j = 1; j <= 3; ++j)
        {
            fr[j] = neww[j];
            ans = min(ans, fr[j]);
        }

    }
    /*
    */
    g << ans;
    f.close();
    g.close();
    return 0;
}