#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <deque>
#include <cmath>
#include <climits>
#include <cstring>
#include <iomanip>
#include <numeric>
#include <functional>
#include <bitset>
#include <unordered_map>
#include <unordered_set>
#include <chrono>
#include <random>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define rep(a,b) for(int a = 0;a<b;a++)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(),a.end()
const int inf = 1e9;
const ll infl = 1e18;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> pref_o(n);
vector<int> pref_i(n);
vector<int> poz_j;
rep(i, n){
if(s[i] == 'J'){
poz_j.pb(i);
}
}
rep(i, n){
if(i == 0){
if(s[0] == 'O'){
pref_o[0] = 1;
}
else if(s[0] == 'I'){
pref_i[0] = 1;
}
}
else{
if(s[i] == 'O'){
pref_o[i] = pref_o[i-1]+1;
}
else{
pref_o[i] = pref_o[i-1];
}
if(s[i] == 'I'){
pref_i[i] = pref_i[i-1]+1;
}
else{
pref_i[i] = pref_i[i-1];
}
}
}
if(poz_j.size() < k){
cout << -1 << "\n";
return 0;
}
int w = inf;
for(int kon = k-1; kon < poz_j.size(); kon++){
int akt = 0;
int poc = kon - (k-1);
int ost_o_liczba;
auto ost_o = lower_bound(pref_o.begin(), pref_o.end(), pref_o[kon]+k);
if(ost_o == pref_o.end()){
continue;
}
else{
ost_o_liczba = ost_o - pref_o.begin();
}
//cout << pref_o[kon]+k << "\n";
//cout << ost_o_liczba << "\n";
auto ost_i = lower_bound(pref_i.begin(), pref_i.end(), pref_i[ost_o_liczba]+k);
if(ost_i == pref_i.end()){
continue;
}
else{
int ost_i_liczba = ost_i - pref_i.begin();
w = min(w, ost_i_liczba-poc+1 - 3*k);
//cout << ost_i_liczba << "\n";
}
}
if(w != inf){
cout << w << "\n";
}
else{
cout << -1 << "\n";
}
return 0;
}
