#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
int n,k; cin>>n>>k;
string s; cin>>s;
vector<int> jpos, opos, ipos;
for(int i=0;i<n;i++) {
if(s[i]=='J')jpos.push_back(i);
if(s[i]=='O')opos.push_back(i);
if(s[i]=='I')ipos.push_back(i);
}
int next_k[n]; memset(next_k, -1, sizeof(next_k));
for(int i=0;i<n;i++) {
if(s[i]=='J') {
int idx = lower_bound(jpos.begin(), jpos.end(), i)-jpos.begin();
if(idx+k-1<jpos.size()) next_k[i] = jpos[idx+k-1];
}
if(s[i]=='O') {
int idx = lower_bound(opos.begin(), opos.end(), i)-opos.begin();
if(idx+k-1<opos.size()) next_k[i] = opos[idx+k-1];
}
if(s[i]=='I') {
int idx = lower_bound(ipos.begin(), ipos.end(), i)-ipos.begin();
if(idx+k-1<ipos.size()) next_k[i] = ipos[idx+k-1];
}
}
int ans=LLONG_MAX;
for(int i=0;i<jpos.size();i++) {
int jstart = jpos[i];
if(next_k[jstart]==-1)break;
int jend = next_k[jstart];
if(lower_bound(opos.begin(),opos.end(),jend)==opos.end()) break;
int ostart = *lower_bound(opos.begin(),opos.end(),jend);
if(next_k[ostart]==-1)break;
int oend = next_k[ostart];
if(lower_bound(ipos.begin(),ipos.end(),oend)==ipos.end()) break;
int istart = *lower_bound(ipos.begin(),ipos.end(),oend);
if(next_k[istart]==-1)break;
int iend = next_k[istart];
ans = min(ans, iend-jstart+1-3*k);
}
if(ans==LLONG_MAX) cout<<-1;
else cout<<ans;
return 0;
}
