#include <iostream>
#include <set>
using namespace std;
const int MAX_TAILLE = 2e5+42, INF = 1e9;
int taille, niveau;
string mot;
int cumul[MAX_TAILLE][3] = {0};
int dicho(int posDeb, int type) {
int deb = posDeb, fin = taille-1;
if(posDeb == -1 || cumul[taille][type] - cumul[posDeb][type] < niveau)
return -1;
while(deb < fin) {
int milieu = (deb + fin) / 2;
if(cumul[milieu+1][type] - cumul[posDeb][type] < niveau) {
deb = milieu+1;
}
else {
fin = milieu;
}
}
return deb;
}
int main() {
cin >> taille >> niveau;
cin >> mot;
for(int cur = 0; cur < taille; cur++) {
if(mot[cur] == 'J')
cumul[cur+1][0]++;
else if(mot[cur] == 'O')
cumul[cur+1][1]++;
else
cumul[cur+1][2]++;
for(int type = 0; type < 3; type++) {
cumul[cur+1][type] += cumul[cur][type];
}
}
int rep = INF;
for(int deb = 0; deb < taille; deb++) {
int posCur = deb;
bool bon = 1;
for(int type = 0; type < 3; type++) {
posCur = dicho(posCur, type);
if(posCur == -1)
bon = 0;
}
if(bon) {
rep = min(rep, posCur - deb - 3 * niveau + 1);
}
}
if(rep == INF) {
cout << "-1";
}
else {
cout << rep;
}
}
