#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using str = string;
using ld = long double;
using hash_map = gp_hash_table<int, int>;
using hash_set = gp_hash_table<int, null_type>;
auto sd = std::chrono::high_resolution_clock::now().time_since_epoch().count();
mt19937 rnd(sd);
using ord_set = tree<ll, null_type, less<>, rb_tree_tag, tree_order_statistics_node_update>;
const ll inf = 1e18;
void solve1() {
ll n, k;
cin >> n >> k;
str s;
cin >> s;
str need;
for(int j = 0; j < k; j++){
need.push_back('J');
}
for (int j =0; j< k; j++){
need.push_back('O');
}
for(int j =0; j< k; j++){
need.push_back('I');
}
deque<char> s1;
for(auto x: s)s1.emplace_back(x);
while (!s1.empty()&& (s1.front()!='J' || s1.back()!='I')){
if (s1.front()!='J'){
s1.pop_front();
}
if (!s1.empty()&& s1.back()!='I'){
s1.pop_back();
}
if (s1.size()<need.size()){
cout<<-1<<endl;
return;
}
}
s ="";
for(auto u : s1)s.push_back(u);
n = s.size();
deque<ll> Js;
deque<ll> Os;
deque<ll> Is;
for (int i=0; i<n ;i++){
if (s[i]=='J'){
Js.push_back(i);
}
else if (s[i]=='O'){
Os.push_back(i);
}
else Is.push_back(i);
}
if (Js.size()<k||Os.size()<k||Is.size()<k){
cout<<-1<<endl;
return;
}
ll mn_ans = inf;
for (int j = k-1; j < Js.size(); j++){
ll cur_j = Js[j];
ll pre_j = Js[j-k+1];
while (!Os.empty()&&Os.front()<cur_j){
Os.pop_front();
}
while (!Is.empty()&&Is.front()<cur_j){
Is.pop_front();
}
if(Is.size()<k||Os.size()<k){
break;
}
ll ans = (cur_j - pre_j+1) - k;
ll pre_o = Os.front();
ans += (pre_o-cur_j-1);
ll cur_o = Os[k-1];
ll nx_it = upper_bound(Is.begin(), Is.end(), cur_o)-Is.begin();
if (nx_it+k-1 >= Is.size()){
break;
}
ll pre_i = Is[nx_it];
ll cur_i = Is[nx_it+k-1];
ans += (cur_o - pre_o+1) - k;
ans += (pre_i-cur_o-1);
ans += (cur_i-pre_i+1)-k;
mn_ans=min(mn_ans, ans);
}
if (mn_ans == inf){
cout<<-1<<endl;
}
else cout << mn_ans<<endl;
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
#ifdef local
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#endif
int t1 = 1;
// cin>>t1;
for (int o_ = 0; o_ < t1; o_++) {
solve1();
}
#ifdef local
printf_s("\n%.5f s", (double) clock() / CLOCKS_PER_SEC);
#endif
}
