#include <bits/stdc++.h>
using namespace std;
#ifndef lisie_bimbi
#define endl '\n'
#pragma GCC optimize("O3")
#pragma GCC target("avx,avx2,bmi2,fma")
#endif
using ll = long long;
const ll inf = 1'000'000'000;
int n, k;
vector<int> J, O, I;
int get(int l, int r, vector<int> &a){
return a[r] - a[l];
}
int getr(int ind, vector<int> &a){
int l = 0, r = n - ind + 1;
while(l + 1 != r){
int m = (l + r) / 2;
if(get(ind, ind + m, a) >= k){
r = m;
}else{
l = m;
}
}
return r;
}
void solve(){
cin >> n >> k;
string s;
cin >> s;
J.resize(n + 1);
O.resize(n + 1);
I.resize(n + 1);
for(int i = 0; i < n; i++){
J[i + 1] = J[i] + (s[i] == 'J');
O[i + 1] = O[i] + (s[i] == 'O');
I[i + 1] = I[i] + (s[i] == 'I');
}
int ans = inf;
for(int i = 0; i < n; i++){
int i2 = i + getr(i, J);
if(i2 == n + 1){
continue;
}
int i3 = i2 + getr(i2, O);
if(i3 == n + 1){
continue;
}
int i4 = i3 + getr(i3, I);
if(i4 == n + 1){
continue;
}
ans = min(ans, i4 - i - 3 * k);
}
if(ans != inf){
cout << ans << endl;
}else{
cout << -1 << endl;
}
}
signed main(){
#ifdef lisie_bimbi
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#else
#endif
cin.tie(nullptr)->sync_with_stdio(false);
int t = 1;
//cin >> t;
while(t--){
solve();
}
}
