#include <bits/stdc++.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
long long n,k;cin >>n >> k;
string s;cin >> s;
//it's best to remove items at the ends
// Do a Sliding window
long long ans = LLONG_MAX;
vector<long long> J;
vector<long long> O;
vector<long long> I;
for(int i = 0;i<n;i++){
if (s[i] == 'J'){
J.push_back(i);
}
if(s[i] == 'O'){
O.push_back(i);
}
if(s[i] == 'I'){
I.push_back(i);
}
}
long long Jpointer = 0;
long long Opointer = 0;
long long Ipointer = 0;
//Note that only the Opointer and Ipointer need to move
map<char,long long> m;
long long index = 0;
for(int i = 0;i<J.size();i++){
Jpointer = J[i];
if (i + k - 1 >= J.size()){
break;
}
long long Jendpointer = J[i+k-1];
while (Opointer < O.size() && O[Opointer] <= Jendpointer){
Opointer += 1;
}
if (Opointer + k - 1 >= O.size()){
continue;
}
long long Oendpointer = O[Opointer+k-1];
while (Ipointer < I.size() && I[Ipointer] <= Oendpointer){
Ipointer += 1;
}
if (Ipointer + k - 1 >= I.size()){
continue;
}
ans = min(ans, I[Ipointer+k-1] - J[i] - 3*k + 1);
}
if (ans == LLONG_MAX){
cout << -1;
return 0;
}
cout << ans;
}
