#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 10'000'000'000'000'000LL;
const int II = 2'000'000'000;
const ll M = 1000'000'007LL;
const int N = 200'007;
vector<int> cur[3];
int gj[N], go[N];
int tp[200];
int Get(int r, int k)
{
if((int)cur[r].size() < k) return -1;
return cur[r][(int)cur[r].size() - k];
}
void Solve()
{
int n, k, ans = II;
string s;
cin >> n >> k;
cin >> s;
tp['J'] = 0; tp['O'] = 1; tp['I'] = 2;
for(int i = 0; i < n; ++i)
{
cur[tp[(int)s[i]]].pb(i);
gj[i] = Get(0, k);
go[i] = Get(1, k);
int l = Get(2, k);
if(l <= 0) continue;
l = go[l - 1];
if(l <= 0) continue;
l = gj[l - 1];
if(l == -1) continue;
ans = min(ans, (i - l + 1) - 3 * k);
}
if(ans < II)
cout << ans << "\n";
else
cout << -1 << "\n";
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//int t; cin >> t;
//while(t--)
Solve();
return 0;
}
