#include "bits/stdc++.h"
using namespace std;
void fast() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
}
void ran() {
srand(chrono::steady_clock::now().time_since_epoch().count());
}
long long get_rand() {
long long a = rand();
long long b = rand();
return a * (RAND_MAX + 1ll) + b;
}
// #define endl '\n'
// #define double long double
// #define int long long
// int MOD = 1000 * 1000 * 1000 + 7;
// int MOD = 998244353;
int n, k;
string s;
const int MXN = 200005;
int pfJ[MXN];
int pfO[MXN];
int pfI[MXN];
int get(int pos) {
// K Js then K Os then K Is
// get the earliest position of that last I
int x = pfJ[pos-1] + k;
int l = lower_bound(pfJ, pfJ+n+1, x) - pfJ;
if (l == n+1) return INT_MAX;
int y = pfO[l] + k;
l = lower_bound(pfO, pfO+n+1, y) - pfO;
if (l == n+1) return INT_MAX;
int z = pfI[l] + k;
l = lower_bound(pfI, pfI+n+1, z) - pfI;
if (l == n+1) return INT_MAX;
return l;
}
signed main() {
ran(); fast();
cin >> n >> k >> s;
for (int i=1; i<=n; i++) {
pfJ[i] = pfJ[i-1];
pfO[i] = pfO[i-1];
pfI[i] = pfI[i-1];
if (s[i-1] == 'J') pfJ[i]++;
if (s[i-1] == 'O') pfO[i]++;
if (s[i-1] == 'I') pfI[i]++;
}
for (int i=n+1; i<MXN; i++) {
pfJ[i] = pfJ[i-1];
pfO[i] = pfO[i-1];
pfI[i] = pfI[i-1];
}
int mn = INT_MAX;
for (int i=1; i<=n; i++) {
// for every 'start'
int e = get(i);
if (e == INT_MAX) continue;
// cout << i << ' ' << e << endl;
mn = min(mn, e - i + 1 - 3 * k);
}
if (mn == INT_MAX) cout << -1 << endl;
else cout << mn << endl;
}
