//gm --- akezhon
#include <bits/stdc++.h>
//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define int long long
#define pb push_back
#define F first
#define S second
#define all(v) v.begin(),v.end()
#define pii pair<int,int>
using namespace std;
int n, k, mn;
string s;
vector <int> pj(200005), po(200005), pi(200005);
void emash(){
cin >> n >> k >> s;
if ( n == 3 ){
if ( k == 1 && s == "JOI" )cout << 0;
else cout << -1;
return;
}
for(int i=0; i < n; i++){
if ( 0 < i ){
pj[i] = pj[i-1];
po[i] = po[i-1];
pi[i] = pi[i-1];
}
if ( s[i] == 'J' )pj[i]++;
if ( s[i] == 'O' )po[i]++;
if ( s[i] == 'I' )pi[i]++;
}
mn = 1e9;
for(int i=0; i < n; i++){
if ( s[i] != 'J' )continue;
int l = i, r = n, f = pj[i]+k-1, res = 0;
while(l <= r){
int m = (l + r) / 2;
if(f <= pj[m]){
res = m;
r = m - 1;
}else{
l = m + 1;
}
}
if ( r == n )continue;
l = res + 1;
r = n;
f = po[l-1]+k;
res = 0;
while(l <= r){
int m = (l + r) / 2;
if(f <= po[m]){
res = m;
r = m - 1;
}else{
l = m + 1;
}
}
if ( r == n )continue;
l = res+1;
r = n;
f = pi[l-1]+k;
res = 0;
while(l <= r){
int m = (l + r) / 2;
if(f <= pi[m]){
res = m;
r = m - 1;
}else{
l = m + 1;
}
}
if ( r == n )continue;
//cout << 3 << ' ' << r << '\n';
mn = min(mn, res-i+1-3*k);
}
cout << (mn == 1e9 ? -1 : mn);
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
//freopen("262144.in", "r", stdin);
//freopen("262144.out", "w", stdout);
int RealName=1;
//cin >> RealName;
int C=0;
while(RealName--){
//cout << "Case " << ++C << ":\n";
emash();
}
return 0;
}
