#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int,int>
#define float long double
#define fi first
#define se second
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(v) (int)v.size()
#define uid uniform_int_distribution<int>
#define forn(i,st,n,inc) for(int i=st;i<n;i+=inc)
#define rforn(i,st,n,inc) for(int i=st-1;i>=n;i-=inc)
//mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
const int MOD = 1e9+7;//998244353;
const int64_t LLINF = 1e18+7;
const int32_t INF = 1e9+7;
ostream& operator<<(ostream& o,const string& s){
for(auto c:s) o<<c;
return o;
}
template<typename F,typename S>
ostream& operator<<(ostream& o,const pair<F,S>& p){
o<<"["<<p.fi<<","<<p.se<<"]";
return o;
}
template<typename... T,template<class...> class C>
ostream& operator<<(ostream& o,const C<T...>& v){
o<<"[";
int tot=0;
for(auto x:v){
o<<x;
if(tot<v.size()-1) o<<",";
tot++;
}
o<<"]";
return o;
}
vector<string> vec_splitter(string s) {
s += ',';
vector<string> res;
while(!s.empty()) {
res.push_back(s.substr(0, s.find(',')));
s = s.substr(s.find(',') + 1);
}
return res;
}
void debug_out(
vector<string> __attribute__ ((unused)) args,
__attribute__ ((unused)) int idx,
__attribute__ ((unused)) int LINE_NUM) { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(vector<string> args, int idx, int LINE_NUM, Head H, Tail... T) {
if(idx > 0) cerr << ", "; else cerr << "Line(" << LINE_NUM << ") ";
stringstream ss; ss << H;
cerr << args[idx] << " = " << ss.str();
debug_out(args, idx + 1, LINE_NUM, T...);
}
#ifdef OFFLINE
clock_t Tm=clock();
#define debug(...) debug_out(vec_splitter(#__VA_ARGS__), 0, __LINE__, __VA_ARGS__)
#else
#define debug(...)
#endif
int32_t main(){
ios_base::sync_with_stdio(false);cin.tie();
//If you hack my code , You are gay
int n,k;
cin>>n>>k;
vector<int> ara(n),cnt(3);
for(int i=0;i<n;i++){
char x;
cin>>x;
if(x=='J') ara[i]=0;
else if(x=='O') ara[i]=1;
else ara[i]=2;
}
vector<int> idx[3];
for(int i=0;i<n;i++) idx[ara[i]].pb(i);
debug(ara);
debug(idx[0]);
debug(idx[1]);
debug(idx[2]);
int ans=n;
for(int j=0;j+k-1<sz(idx[0]);j++){
int o=upper_bound(all(idx[1]),idx[0][j+k-1])-idx[1].begin();
if(o+k-1>=sz(idx[1])) break;
int i=upper_bound(all(idx[2]),idx[1][o+k-1])-idx[2].begin();
if(i+k-1>=sz(idx[2])) break;
debug(j,o,i);
int now=(idx[2][i+k-1]-idx[0][j]+1)-3*k;
ans=min(ans,now);
}
if(ans==n) ans=-1;
cout<<ans;
//mara kha
#ifdef OFFLINE
cerr<<"Time = "<<(double)(clock()-Tm)/CLOCKS_PER_SEC;
#endif
return 0;
}
