#include <bits/stdc++.h>
using namespace std;
int minim[2][300001];
char sir[300001];
int main ()
{
ios :: sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int lungime , cantitate;
cin >> lungime >> cantitate >> sir;
for (auto dorit : {'J' , 'O' , 'I'})
{
queue <int> gasit;
for (int indice = 0 ; indice <= lungime ; indice++)
{
if (sir[indice - 1] == dorit)
{ gasit.push(indice); }
if ((int)gasit.size() > cantitate)
{ gasit.pop(); }
if ((int)gasit.size() < cantitate || minim[0][gasit.front() - 1] == -1)
{ minim[1][indice] = -1; continue; }
minim[1][indice] = minim[0][gasit.front() - 1] + indice - gasit.front() + 1;
}
for (int indice = 0 ; indice <= lungime ; indice++)
{ minim[0][indice] = minim[1][indice]; }
}
if (minim[1][lungime] == -1)
{ cout << "-1"; return 0; }
int rezultat = INT32_MAX;
for (int indice = 0 ; indice <= lungime ; indice++) {
if (minim[1][indice] != -1)
{ rezultat = min(rezultat , minim[1][indice]); }
}
cout << rezultat - 3 * cantitate;
return 0;
}
