#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define dd(x) cout<<#x<<" is "<<x<<endl;
#define dd2(x,y) cout<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<endl;
typedef pair<int,int> pii;
struct dat{
int l,r;
} os[200005];
signed main(){
int n,k; cin>>n>>k;
string s; cin>>s;
deque<int> js, is, oids;
for(int i=0; i<n; i++){
if(s[i] == 'J'){
js.pb(i);
if(js.size() > k) js.pop_front();
}
if(s[i] == 'O'){
os[i].l = (js.size() == k)? js.front() : -1;
oids.pb(i);
}
}
for(int i=n-1; i>=0; i--){
if(s[i] == 'I'){
is.push_front(i);
if(is.size() > k) is.pop_back();
}
if(s[i] == 'O'){
os[i].r = (is.size() == k)? is.back() : -1;
}
}
int ans = LLONG_MAX, num = oids.size();
for(int x=0; x<=num-k; x++){
int i=oids[x], j=oids[x+k-1];
if(os[i].l == -1 or os[j].r == -1) continue;
ans = min(ans,os[j].r - os[i].l + 1 - 3*k);
}
cout<< ((ans==LLONG_MAX)? -1 : ans);
return 0;
}
