#include <bits/stdc++.h>
using namespace std;
#define int long long
const int inf = 1e9;
signed main(){
ios::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
int n, k, ans = inf;
string str;
cin >> n >> k >> str;
vector<int> dpje(n + 5, inf), dpof(n + 5, inf), dpoe(n + 5, inf), dpif(n + 5, inf);
vector<char> pos[30];
for(int i = 0; i < (int) str.size(); ++i){
int key = str[i] - 'A';
pos[key].push_back(i);
if(str[i] == 'J'){
if((int) pos[key].size() >= k){
dpje[i] = i - pos[key][(int) pos[key].size() - k] + 1 - k;
}
}else if(str[i] == 'O'){
if((int) pos['J' - 'A'].size()){
if(i) dpof[i] = dpof[pos[key].back()] + i - pos[key].back();
dpof[i] = min(dpof[i], dpje[pos['J' - 'A'].back()] + i - pos['J' - 'A'].back() - 1);
}
if((int) pos[key].size() >= k){
if(i) dpoe[i] = dpoe[pos[key].back()] + i - pos[key].back();
dpoe[i] = min(dpoe[i], dpof[pos[key][(int) pos[key].size() - k]] + i - pos[key][(int) pos[key].size() - k] + 1 - k);
}
// cout << dpof[i] << " " << dpoe[i] << "\n";
}else{
if((int) pos['O' - 'A'].size()){
if(i) dpif[i] = dpif[pos[key].back()] + i - pos[key].back();
dpif[i] = min(dpif[i], dpoe[pos['O' - 'A'].back()] + i - pos['O' - 'A'].back() - 1);
}
if((int) pos[key].size() >= k){
ans = min(ans, dpif[pos[key][(int) pos[key].size() - k]] + i - pos[key][(int) pos[key].size() - k] + 1 - k);
}
}
}
cout << (ans >= ((int) str.size()) ? -1 : ans) << "\n";
return 0;
}
