#include <iostream>
#include <vector>
using namespace std;
string s;
int n,k;
vector<int> a,b,c;
vector<int> calc(char c){
vector<int> a(n+2);
int j=0,j1=0,i;
for(i=0;i<n;i++){
if(i && s[i-1]==c) j1--;
while(j<n && j1<k){
if(s[j]==c) j1++;
j++;
}
if(j1==k) a[i]=j;
else a[i]=n+1;
}
a[n]=a[n+1]=n+1;
return a;
}
int main(){
int rasp,i;
cin>>n>>k>>s;rasp=n;
a=calc('J');b=calc('O');c=calc('I');
for(i=0;i<n;i++){
if(c[b[a[i]]]<=n) rasp=min(rasp,c[b[a[i]]]-i-3*k);
}
if(rasp==n) cout<<"-1";
else cout<<rasp;
}
