#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
int n, k, pf[N], sf[N];
string s;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> k >> s;
s = " " + s;
deque<int> dq;
memset(pf, -1, sizeof pf); memset(sf, -1, sizeof sf);
for(int i = 1;i<=n;i++){
if(s[i] == 'J') dq.push_front(i);
while(dq.size() > k) dq.pop_back();
if(dq.size() == k) pf[i] = dq.back();
}
while(dq.size()) dq.pop_back();
for(int i = n;i>=1;i--){
if(s[i] == 'I') dq.push_front(i);
while(dq.size() > k) dq.pop_back();
if(dq.size() == k) sf[i] = dq.back();
}
while(dq.size()) dq.pop_back();
int ans = INT_MAX;
for(int i = 1;i<=n;i++){
if(s[i] == 'O') dq.push_front(i);
while(dq.size() > k) dq.pop_back();
if(dq.size() == k && pf[dq.back() - 1] != -1 && sf[i + 1] != -1){
ans = min(ans, sf[i + 1] - pf[dq.back() - 1] + 1 - 3 * k);
}
}
cout << (ans == INT_MAX ? -1 : ans);
}
