#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define INF 1e9+7
#define all(x) x.begin(),x.end()
using namespace std;
using namespace __gnu_pbds;
using ll=long long;
using pii=pair<int,int>;
using ppi=pair<int,pii>;
using oset=tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>;
template<typename T>
void _print(vector<T> x) {for(auto e:x) cerr<<e<<",";}
template<typename T>
void _print(T x) {cerr<<x;}
void dbg() {cerr<<endl;}
template<typename Head,typename... Tail>
void dbg(Head H,Tail... T) {
_print(H);
if(sizeof...(T)) cerr<<",";
else cerr<<"\"]";
dbg(T...);
}
#define debug(...) cerr<<"["<<#__VA_ARGS__<<"]:[\"",dbg(__VA_ARGS__)
int n,k;
string s;
int main () {
ios::sync_with_stdio(false); cin.tie(0);
cin>>n>>k>>s;
vector<int> J,O,I;
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);
}
int j_len=J.size(),o_len=O.size(),i_len=I.size();
if(j_len<k or o_len<k or i_len<k) {
cout<<"-1"<<endl; return 0;
}
int ans=INF;
//debug(J); debug(O); debug(I);
for(int i=0;i<o_len-k+1;i++) {
int first_O=O[i];
int last_O=O[i+k-1];
int pos1_I=lower_bound(all(I),last_O)-begin(I);
if(I[pos1_I]<last_O or pos1_I+k>i_len) continue;
int last_all=I[pos1_I+k-1];
int pos2_J=lower_bound(all(J),first_O)-begin(J)-1;
if(pos2_J<k-1 or J[pos2_J]>first_O) continue;
int first_all=J[pos2_J-k+1];
ans=min(ans,last_all-first_all-3*k+1);
}
if(ans==INF) cout<<"-1"<<endl;
else cout<<ans<<endl;
}
