#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for(int i=a;i<b;++i)
int n,k,result;
string s;
signed main(){
cin>>n>>k>>s;
s='*'+s;
vector<int> vec1,vec2,vec3;
rep(i,1,n+1){
if(s[i]=='J') vec1.push_back(i);
if(s[i]=='O') vec2.push_back(i);
if(s[i]=='I') vec3.push_back(i);
}
result=LLONG_MAX;
rep(j,0,vec1.size()-k+1){
int j_end=vec1[j+k-1];
auto o_it=lower_bound(vec2.begin(),vec2.end(),j_end+1);
if(o_it+k-1>=vec2.end()) continue;
int o_end=*(o_it+k-1);
auto i_it=lower_bound(vec3.begin(),vec3.end(),o_end+1);
if(i_it+k-1>=vec3.end()) continue;
int i_end=*(i_it+k-1);
int length=i_end-vec1[j]+1;
int cost=length-3*k;
result=min(result,cost);
}
if(result==LLONG_MAX) cout<<-1<<endl;
else cout<<result<<endl;
}
