#ifndef LOCAL
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC diagnostic ignored "-Wpedantic"
#endif
#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;
auto seed = chrono::high_resolution_clock::now().time_since_epoch().count();
mt19937 rnd(seed);
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename V>
using table = gp_hash_table<T, V>;
using i128 = __int128;
using ll = long long;
using ld = long double;
using uint = unsigned int;
using ull = unsigned long long;
const ll INF = 1e13;
const int inf = 1e9;
const int maxk = 140;
const int MOD = 1e9 + 7;
const double pi = acos(-1);
const int P = 5167;
const int L = 26;
const ld EPS = 1e-7;
const int W = 3;
const int MBIT = 18;
const int maxn = 2e5 + 7;
const int B = 550;
template<typename T, typename V>
void fill(T &container, V value) {
for (auto &c : container)
c = value;
}
# define int ll
void solve() {
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> J(n), O(n), I(n);
deque<int> jj, oo, ii;
for (int i = n - 1; i >= 0; --i) {
if (s[i] == 'J') jj.push_front(i);
if (s[i] == 'O') oo.push_front(i);
if (s[i] == 'I') ii.push_front(i);
if (jj.size() < k) J[i] = -1;
else J[i] = jj[k - 1];
if (oo.size() < k) O[i] = -1;
else O[i] = oo[k - 1];
if (ii.size() < k) I[i] = -1;
else I[i] = ii[k - 1];
}
int ans = INF;
for (int l = 0; l < n; ++l) {
int j = J[l];
if (j != -1) {
int o = O[j];
if (o != -1) {
int i = I[o];
if (i != -1) {
ans = min(ans, i - l + 1 - 3 * k);
}
}
}
}
if (ans == INF) ans = -1;
cout << ans;
}
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout << fixed << setprecision(9);
int t = 1;
//cin >> t;
while (t--) solve();
#ifdef LOCAL
cout << "\n\n\n=====================================================\nProgram worked for: ";
cout << (ld) clock() / CLOCKS_PER_SEC << "s";
#endif
//stress();
}
