/* made by amunduzbaev */
//~ #include <ext/pb_ds/assoc_container.hpp>
//~ #include <ext/pb_ds/tree_policy.hpp>
#include "bits/stdc++.h"
using namespace std;
//~ using namespace __gnu_pbds;
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define ub upper_bound
#define lb lower_bound
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(),x.rend()
#define NeedForSpeed ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define vv vector
#define mem(arr, v) memset(arr, v, sizeof arr)
#define int long long
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef vector<int> vii;
typedef vector<pii> vpii;
template<class T> bool umin(T& a, const T& b) { return a > b ? a = b, true : false; }
template<class T> bool umax(T& a, const T& b) { return a < b ? a = b, true : false; }
template<int sz> using tut = array<int, sz>;
void usaco(string s) { freopen((s+".in").c_str(),"r",stdin);
freopen((s+".out").c_str(),"w",stdout); NeedForSpeed }
//~ template<class T> using oset = tree<T,
//~ null_type, less_equal<T>, rb_tree_tag,
//~ tree_order_statistics_node_update> ordered_set;
const int N = 2e5+5;
const int mod = 1e9+7;
const ll inf = 1e18;
const ld Pi = acos(-1);
#define MULTI 0
int n, m, k, s, t, q, ans, res, a[N];
int pref[3][N];
void solve(int t_case){
string s;
cin>>n>>k>>s;
vii tt[3];
for(int i=0;i<n;i++){
if(s[i] == 'J') tt[0].pb(i);
if(s[i] == 'O') tt[1].pb(i);
if(s[i] == 'I') tt[2].pb(i);
}
if(min({sz(tt[0]), sz(tt[1]), sz(tt[2])}) < k) { cout<<-1<<"\n"; return; }
deque<int> ss;
int res = inf;
auto check = [&]() -> void{
auto lx = ub(all(tt[0]), ss.front());
auto rx = lb(all(tt[2]), ss.back());
if(lx != tt[0].begin() && rx != tt[2].end()){
int l = lx - tt[0].begin() - 1, r = rx - tt[2].begin();
if(r+k-1 >= sz(tt[2]) || l-k+1 < 0) return;
//~ cout<<l-k+1<<tt[0][l]<<" "<<tt[2][r]<<"\n";
//~ cout<<tt[2][r+k-1]<<" "<<tt[0][l-k+1]<<"\n";
int cost = tt[2][r+k-1] - tt[0][l-k+1] + 1 - 3 * k;
umin(res, cost);
}
};
/*
9 2
JOJOIOIJI
*/
for(int i=0;i<k;i++) ss.pb(tt[1][i]);
check();
for(int i=k;i<sz(tt[1]);i++) ss.pop_front(), ss.pb(tt[1][i]), check();
if(res == inf) cout<<-1<<"\n";
else cout<<res<<"\n";
}
signed main(){
NeedForSpeed
if(MULTI){
int t; cin>>t;
for(int t_case = 1; t_case <= t; t_case++) solve(t_case);
} else solve(1);
return 0;
}
