#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#define fast ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
#define file(s) freopen(s".in", "r", stdin);freopen(s".out", "w", stdout);
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define pb push_back
#define sz size()
#define ft first
#define sd second
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef unsigned long long ull;
const int N = 1e6 + 10;
const int M = 1e3 + 5;
const ll inf = 1e9;
const ll mod = 1e9 + 7;
const double Pi = acos(-1);
ll binpow(ll x, ll ti) { ll res = 1;while (ti){if(ti & 1)res *= x;x *= x;ti >>= 1; x %= mod; res %= mod;} return res;}
ll binmul(ll x, ll ti) { ll res = 0;while (ti){if(ti & 1)res += x;x += x;ti >>= 1; x %= mod; res %= mod;} return res;}
ll nok(ll a, ll b) { return (a*b)/__gcd(abs(a),abs(b)) * (a*b > 0 ? 1 : -1); }
bool odd(ll n) { return (n % 2 == 1); }
bool even(ll n) { return (n % 2 == 0); }
ll n, k, a[N], b[N], c[N];
string s;
const void solve(/*Armashka*/) {
cin >> n >> k >> s;
s = " " + s;
ll ans = inf;
for (int i = 1; i <= n; ++ i) {
a[i] = a[i - 1];
b[i] = b[i - 1];
c[i] = c[i - 1];
if (s[i] == 'J') ++ a[i];
else if (s[i] == 'O') ++ b[i];
else ++ c[i];
}
for (int i = 1; i <= n; ++ i) {
int l = i, r = n, j = -1;
while (l <= r) {
int mid = (l + r) / 2;
if (a[mid] - a[i - 1] >= k) r = mid - 1, j = mid;
else l = mid + 1;
}
if (j == -1) continue;
l = j + 1, r = n;
int kk = -1;
while (l <= r) {
int mid = (l + r) / 2;
if (b[mid] - b[j] >= k) r = mid - 1, kk = mid;
else l = mid + 1;
}
if (kk == -1) continue;
l = kk + 1, r = n;
int h = -1;
while (l <= r) {
int mid = (l + r) / 2;
if (c[mid] - c[kk] >= k) r = mid - 1, h = mid;
else l = mid + 1;
}
if (h == -1) continue;
ans = min(ans, (h - i + 1) - k * 3);
}
if (ans == inf) ans = -1;
cout << ans;
}
signed main() {
srand(time(NULL));
fast;
// file("");
int tt = 1;
// cin >> tt;
while (tt --) {
solve();
}
}
// Jelbiretip kok tudy, boys in the hood
// Kim qandai juz, kimde qandai ru
