#include <bits/stdc++.h>
#define ff first
#define ss second
#define endl '\n'
using namespace std;
const long long INF = (long long) 1e18;
const int mod = (int) 1e9+7;
const int MAXN = (int) 3e5+5;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
ll n, k;
string s;
vector<int> j, o, I;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
#ifdef Local
freopen("C:/Users/Admin/Desktop/Yazilim/C/IO/int.txt","r",stdin);
freopen("C:/Users/Admin/Desktop/Yazilim/C/IO/out.txt","w",stdout);
#endif
cin>>n>>k;
int ans = mod;
cin>>s;
for(int i = 0; i < n; i++){
if(s[i] == 'J') j.push_back(i);
else if(s[i] == 'O') o.push_back(i);
else I.push_back(i);
}
for(int i = 0; i <= (int) o.size() - k; i++){
int i1 = upper_bound(j.begin(), j.end(), o[i]) - j.begin() - 1;
int i2 = upper_bound(I.begin(), I.end(), o[i + k - 1]) - I.begin();
if(i1 < k - 1 || i2 > (int) I.size() - k) continue;
int sim = (j[i1] - j[i1 - k + 1] + 1 - k);
sim += o[i] - j[i1] - 1;
sim += o[i + k - 1] - o[i] + 1 - k;
sim += I[i2] - o[i + k - 1] - 1;
sim += I[i2 + k - 1] - I[i2] + 1 - k;
//cout<<sim<<endl;
ans = min(ans, sim);
}
if(ans == mod)
ans = -1;
cout<<ans<<endl;
#ifdef Local
cout<<endl<<fixed<<setprecision(2)<<1000.0 * clock() / CLOCKS_PER_SEC<< " milliseconds ";
#endif
}
