#include <bits/stdc++.h>
#pragma GCC optimize ("O2,unroll-loops")
//#pragma GCC optimize("no-stack-protector,fast-math")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef pair<ll, ll> pll;
#define debug(x) cerr<<#x<<'='<<(x)<<endl;
#define debugp(x) cerr<<#x<<"= {"<<(x.first)<<", "<<(x.second)<<"}"<<endl;
#define debug2(x, y) cerr<<"{"<<#x<<", "<<#y<<"} = {"<<(x)<<", "<<(y)<<"}"<<endl;
#define debugv(v) {cerr<<#v<<" : ";for (auto x:v) cerr<<x<<' ';cerr<<endl;}
#define all(x) x.begin(), x.end()
#define pb push_back
#define kill(x) return cout<<x<<'\n', 0;
const ld eps=1e-7;
const int inf=1000000010;
const ll INF=10000000000000010LL;
const int mod=1000000007;
const int MAXN=100010, LOG=20;
int n, m, k, u, v, x, y, t, a, b, ans=inf;
int A[MAXN];
vector<int> J, O, I;
string S;
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
cin>>n>>k>>S;
for (int i=0; i<n; i++){
if (S[i]=='J') J.pb(i);
if (S[i]=='O') O.pb(i);
if (S[i]=='I') I.pb(i);
}
J.pb(inf);
O.pb(inf);
I.pb(inf);
for (int i=0; i<n; i++) if (S[i]=='J'){
int pos=i, tmp;
tmp=lower_bound(all(J), pos)-J.begin();
if (tmp+k-1>=J.size()-1) continue ;
pos=J[tmp+k-1]+1;
tmp=lower_bound(all(O), pos)-O.begin();
if (tmp+k-1>=O.size()-1) continue ;
pos=O[tmp+k-1]+1;
tmp=lower_bound(all(I), pos)-I.begin();
if (tmp+k-1>=I.size()-1) continue ;
pos=I[tmp+k-1]+1;
ans=min(ans, pos-i-3*k);
}
if (ans>n) ans=-1;
cout<<ans<<'\n';
return 0;
}
