#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(),a.end()
int n, k;
string s;
vector<int> v[26];
int main() {
cin >> n >> k >> s; int ans = 1000000;
for(int i = 0; i < n; i++)
v[s[i]-'A'].pb(i);
for(int i = 0; i < 26; i++)
if(!v[i].empty()) sort(all(v[i]));
for(int i = 0; i < n; i++){
if(s[i]!='J') continue;
int J = 'J'-'A', O = 'O'-'A', I = 'I'-'A';
auto itr = lower_bound(all(v[J]),i)-begin(v[J]);
if(itr+k-1>=sz(v[J])) break;
int posJ = v[J][itr+k-1];
itr = lower_bound(all(v[O]),posJ)-begin(v[O]);
if(itr+k-1>=sz(v[O])) break;
int posO = v[O][itr+k-1];
itr = lower_bound(all(v[I]),posO)-begin(v[I]);
if(itr+k-1>=sz(v[I])) break;
int posI = v[I][itr+k-1];
ans = min(ans, posI-i+1-3*k);
}
cout << (ans==1000000?-1:ans);
}
