#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N = 2e5 + 11;
const int inf = 1e18 + 7;
vector < int > pos[91];
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
int n, k;
string s;
cin >> n >> k >> s;
for(int i = 0; i < n; i++){
pos[s[i]].push_back(i);
}
int ans = n + 1;
for(int i = 0; i < n; i++){
if(s[i] != 'J') continue;
int J = lower_bound(pos['J'].begin(), pos['J'].end(), i) - pos['J'].begin();
if(J + k - 1 >= pos['J'].size()) break;
int j = pos['J'][J + k - 1];
int O = lower_bound(pos['O'].begin(), pos['O'].end(), j) - pos['O'].begin();
if(O + k - 1 >= pos['O'].size()) break;
j = pos['O'][O + k - 1];
int I = lower_bound(pos['I'].begin(), pos['I'].end(), j) - pos['I'].begin();
if(I + k - 1 >= pos['I'].size()) break;
j = pos['I'][I + k - 1];
ans = min(ans, j - i + 1 - 3 * k);
}
if(ans == n + 1) ans = -1;
cout << ans;
}
// subete no mono no owari wa sugu ni yattekuru
