#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define MOD 998244353
void solve()
{
ll n, k;
string str;
cin >> n >> k >> str;
vector<vector<ll>> pos(3);
vector<ll> last(4, -1e9);
map<char, ll> id = {{'J', 0}, {'O', 1}, {'I', 2}};
ll ans = 1e9;
for (ll i = 0; i < n; i++)
{
last[0] = i;
ll x = id[str[i]];
pos[x].emplace_back(last[x]);
if (pos[x].size() >= k)
last[x + 1] = pos[x][pos[x].size() - k];
ans = min(ans, i - last[3] + 1 - 3 * k);
}
if (ans > n)
ans = -1;
cout << ans << "\n";
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
ll t = 1;
// cin >> t;
// cout << fixed << setprecision(12);
for (ll i = 1; i <= t; i++)
{
solve();
}
}
