#include <bits/stdc++.h>
using namespace std;
#define kien long long
#define int long long
#define pb push_back
#define FOR(i, a, b) for (int i = a;i <= b; i++)
#define FORD(i, a, b) for (int i = a;i >= b; i--)
#define pii pair<int, int>
#define dembit(a) __builtin_popcountll(a)
#define task "kien"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define debug(x) cout << x << " "
#define down cout << "\n"
const kien MOD = 1e18 + 7;
const int NTEST = 100;
const int Million = 1e6;
const int MXN = 2e5 + 5;
const int INF = 1e9;
mt19937 rd(chrono::high_resolution_clock::now().time_since_epoch().count());
kien rand(kien l, kien r) {
assert(l <= r);
return uniform_int_distribution<kien>(l, r)(rd);
}
kien n, k, m, u, v, h, xuong[MXN], pre[MXN], len[MXN];
kien maxx, minn, sum, vtr, s, t, r, x, l, timer, ans, add;
vector <int> dau, cuoi, mid;
int tknp (int kk) {
int l = 0, r = dau.size() - 1;
int kq = -1;
while (l <= r) {
int mid = (l + r) >> 1;
if (dau[mid] <= kk) l = mid + 1, kq = mid;
else r = mid - 1;
}
return kq;
}
int tknp1 (int kk) {
int l = 0, r = cuoi.size() - 1;
int kq = -1;
while (l <= r) {
int mid = (l + r) >> 1;
if (cuoi[mid] >= kk) l = mid + 1, kq = mid;
else r = mid - 1;
}
return kq;
}
void solve() {
cin >> n >> k;
string s; cin >> s;
FOR (i, 0, s.size() - 1) {
if (i > 0)
pre[i] = pre[i - 1];
if (s[i] == 'J') dau.pb(i);
if (s[i] == 'O') ++pre[i], mid.pb(i);
}
FORD (i, s.size() - 1, 0) {
// suf[i] = suf[i + 1];
if (s[i] == 'I') cuoi.pb(i);
}
if (dau.size() < k or cuoi.size() < k or mid.size() < k) {
cout << "-1\n"; return;
}
int dem = 0;
FOR (i, 0, dau.size() - 1) {
if (k == 1) {len[i] = 0; continue;}
if (i < k) {if (i > 0) len[i] = len[i - 1] + (dau[i] - dau[i - 1] - 1);}
else {
len[i] = (len[i - 1] - len[i - k + 1]) + (dau[i] - dau[i - 1] - 1);
}
}
FOR (i, 0, cuoi.size() - 1) {
if (k == 1) {xuong[i] = 0; continue;}
if (i < k) {if (i > 0) xuong[i] = xuong[i - 1] + (cuoi[i - 1] - cuoi[i] - 1);}
else {
xuong[i] = (xuong[i - 1] - xuong[i - k + 1]) + (cuoi[i - 1] - cuoi[i] - 1);
}
// cout << xuong[i] << " ";
}
FOR (i, 0, k - 2) len[i] = xuong[i] = -1;
int dd = 0, cc; ans = INF;
FOR (i, k - 1, mid.size() - 1) {
cc = i; //cout << mid[cc] << " ";
int tren = tknp1(mid[cc]); int duoi = tknp(mid[dd]);
++dd;
if (tren == -1 or duoi == -1) continue;
if (len[duoi] == -1 or xuong[tren] == -1 or pre[cuoi[tren]] - pre[dau[duoi] - 1] < k) continue;
ans = min(ans, len[duoi] + xuong[tren] + (cuoi[tren] - dau[duoi] - 1 - k));
}
if (ans == INF) {cout << -1; return;}
cout << ans << "\n";
}
main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
if (fopen(task".inp", "r")) {
fin(task); fou(task);
}
int tt = 1; // cin >> tt;
while(tt--) {
solve();
}
cerr << "\n" << 1.0 * clock() / CLOCKS_PER_SEC << "s ";
}
