#include <bits/stdc++.h>
#define pb push_back
#define ll long long
#define f first
#define s second
using namespace std;
const ll N = 2e6 + 7;
const ll inf = 1e9;
const ll INF = 1e18;
const ll MOD = 1e9 + 7;
void be(){
ios_base::sync_with_stdio (0);
cin.tie (0);
// freopen("herding.in","r",stdin);
// freopen("herding.out","w",stdout);
}
void solve (){
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector <int> a(n), b(n), c(n);
for (int i = 0; i < n; ++i){
if (i) a[i] = a[i - 1], b[i] = b[i - 1], c[i] = c[i - 1];
if (s[i] == 'J') ++a[i];
if (s[i] == 'O') ++b[i];
if (s[i] == 'I') ++c[i];
}
int mn = inf;
// for (int i = 0; i < n; ++i) cout << a[i] << ' ';
// cout << '\n';
// for (int i = 0; i < n; ++i) cout << b[i] << ' ';
// cout << '\n';
// for (int i = 0; i < n; ++i) cout << c[i] << ' ';
// cout << '\n';
bool ok = 0;
int pos1, pos2, pos3;
pos1 = lower_bound (a.begin (), a.end (), k) - a.begin ();
if (pos1 == n) ok = 1;
if (!ok) pos2 = lower_bound (b.begin () + pos1, b.end (), k + b[pos1]) - b.begin ();
if (pos2 == n) ok = 1;
if (!ok) pos3 = lower_bound (c.begin () + pos2, c.end (), k + c[pos2]) - c.begin ();
if (pos3 == n) ok = 1;
// cout << pos1 << ' ' << pos2 << ' ' << pos3 << '\n';
if (!ok) mn = min (mn, (pos3 + 1 - 3 * k));
for (int i = 1; i < n; ++i){
pos1 = lower_bound (a.begin () + i, a.end (), k + a[i - 1]) - a.begin ();
if (pos1 == n) break;
pos2 = lower_bound (b.begin () + pos1, b.end (), k + b[pos1]) - b.begin ();
if (pos2 == n) break;
pos3 = lower_bound (c.begin () + pos2, c.end (), k + c[pos2]) - c.begin ();
if (pos3 == n) break;
mn = min (mn, (pos3 - i - 3 * k + 1));
// cout << i << ' ' << pos1 << ' ' << pos2 << ' ' << pos3 << '\n';
}
if (mn == inf) mn = -1;
cout << mn;
}
main (){
be ();
ll tt = 1;
// cin >> tt;
for (ll i = 1; i <= tt; ++i){
// cout << "Case " << i << ':';
solve ();
}
return 0;
}
