#include <bits/stdc++.h>
using namespace std;
typedef long long lo;
#define fi first
#define se second
#define endl "\n"
#define pb push_back
#define fio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define FOR for(int i=1;i<=n;i++)
#define mid ((start+end)/2)
#define ort ((bas+son)/2)
const lo inf = 1000000000;
const lo li = 500005;
const lo mod = 1000000007;
int n,m,a[li],k,flag,t;
int cev;
string s;
vector<int> v[3];
inline int in(){
int x;
scanf("%d",&x);
return x;
}
int main(void){
fio();
cin>>t>>k>>s;
n=s.size();
for(int i=0;i<n;i++){
if(s[i]=='J')v[0].pb(i);
if(s[i]=='O')v[1].pb(i);
if(s[i]=='I')v[2].pb(i);
}
int siz=v[0].size();
cev=n+1;
for(int i=0;i<siz;i++){
int at=i+k-1;
if(at>=siz)break;
int tut=v[0][at];
int ind=upper_bound(v[1].begin(),v[1].end(),tut)-v[1].begin();
ind+=k-1;
if(ind>=(int)v[1].size()){break;}
tut=v[1][ind];
ind=upper_bound(v[2].begin(),v[2].end(),tut)-v[2].begin();
ind+=k-1;
if(ind>=(int)v[2].size())break;
cev=min(cev,v[2][ind]-v[0][i]+1-3*k);
}
if(cev==n+1)cev=-1;
cout<<cev<<endl;
return 0;
}
