//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
template <class T>
using Tree =
tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define int long long
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second
#define rep(i, a, b) for (int i = a; i <= b; ++i)
#define per(i, b, a) for (int i = b; i >= a; --i)
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(), (v).end()
#define lsb(x) (x)&(-x)
#define bug(x) cerr << #x << " " << x << endl;
void setIO(string name = "") {
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
if (!name.empty()) {
freopen((name + ".in").c_str(), "r", stdin);
freopen((name + ".out").c_str(), "w", stdout);
}
}
ll fexp(ll a, ll b, ll m) {
if (b == 0) return 1LL;
ll p = a;
ll ans = 1;
while (b > 0) {
if (b % 2 != 0) ans = (ans * p) % m;
p = (p * p) % m;
b >>= 1;
}
return ans;
}
const int MAXN = 2e5 + 10;
const int INF = 1e18 + 5;
const int MOD = 1e9+7;
int a[MAXN], pfx[MAXN][3],n,k;
int hihi(int i, int j) {
int lo = i+1, hi = n, ans = n+1;
while (lo <= hi) {
int mid = (lo+hi)/2;
if (pfx[mid][j] - pfx[i][j] >= k) {
ans = mid;
hi = mid - 1;
} else {
lo = mid + 1;
}
}
return ans;
}
void solve() {
string s;
cin >> n >> k >> s;
rep(i,1,n) {
if (s[i-1]=='J') a[i] = 0;
else if (s[i-1] == 'O') a[i] = 1;
else a[i] = 2;
pfx[i][a[i]]++;
rep(j,0,2) pfx[i][j] += pfx[i-1][j];
}
int ans = INF;
rep(l,1,n) {
int sz = hihi(l-1,0);
sz = hihi(sz,1);
sz = hihi(sz,2);
if (sz <= n) ans = min(ans, sz-l+1-3*k);
}
cout << (ans==INF?-1:ans) << '\n';
}
int32_t main() {
setIO();
int tt = 1;
//cin >> tt;
while (tt-- > 0) solve();
return 0;
}
