#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define FOR(i, a, b) for(ll i = (ll)a; i <= (ll)b; i++)
#define DEC(i, a, b) for(ll i = (ll)a; i >= (ll)b; i--)
typedef pair<ll, ll> pi;
typedef pair<pi, ll> pii;
typedef pair<ll, pi> ipi;
typedef pair<pi, pi> pipi;
#define f first
#define s second
typedef vector<ll> vi;
typedef vector<pi> vpi;
typedef vector<pii> vpii;
#define pb push_back
#define pf push_front
#define all(v) v.begin(), v.end()
#define size(v) (ll) v.size()
#define disc(v) sort(all(v)); v.resize(unique(all(v)) - v.begin());
#define INF (ll) 1e9 + 100
#define LLINF (ll) 1e18
#define fastio ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define sandybridge __attribute__((optimize("Ofast"), target("arch=sandybridge")))
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count()); //can be used by calling rng() or shuffle(A, A+n, rng)
inline ll rand(ll x, ll y) { ++y; return (rng() % (y-x)) + x; } //inclusivesss
ll n, k, ans = INF;
char ch;
vi J, O, I;
int main() {
fastio; cin >> n >> k;
FOR(i, 1, n) {
cin >> ch;
if (ch == 'J') J.pb(i);
else if (ch == 'O') O.pb(i);
else I.pb(i);
}
FOR(i, 0, size(J)-k) {
ll j = i + k - 1;
ll opos = lower_bound(all(O), J[j]) - O.begin();
if (opos + k - 1 < size(O)) {
ll ipos = lower_bound(all(I), O[opos + k - 1]) - I.begin();
if (ipos + k - 1 < size(I)) ans = min(ans, I[ipos + k - 1] - J[i] + 1 - 3*k);
}
}
cout << (ans == INF ? -1 : ans);
}
