#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define endl "\n"
using namespace std;
using namespace __gnu_pbds;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
template <typename T, typename key = less_equal<T>>
using ordered_set = tree<T, null_type, key, rb_tree_tag, tree_order_statistics_node_update>;
void solve()
{
ll n, k;
cin >> n >> k;
string s;
cin >> s;
ll ans = 1e9;
vector<ll> v[3];
for (ll i = 0; i < n; i++)
{
if (s[i] == 'J') s[i] = 0;
else if (s[i] == 'O') s[i] = 1;
else s[i] = 2;
v[s[i]].push_back(i);
}
for (ll i = 0; i + k - 1 < v[0].size(); i++)
{
ll pos = lower_bound(v[1].begin(), v[1].end(), v[0][i + k - 1] + 1) - v[1].begin();
if (pos + k - 1 >= v[1].size()) continue;
pos = lower_bound(v[2].begin(), v[2].end(), v[1][pos + k - 1] + 1) - v[2].begin();
if (pos + k - 1 >= v[2].size()) continue;
ans = min(ans, v[2][pos + k - 1] - v[0][i] + 1 - 3 * k);
}
if (ans == 1e9) ans = -1;
cout << ans << endl;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
ll t = 1;
// precomp();
// cin >> t;
for (ll cs = 1; cs <= t; cs++)
solve();
// cerr << "\nTime elapsed: " << clock() * 1000.0 / CLOCKS_PER_SEC << " ms\n";
}
