#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
int n, k;
string s;
vector<int> J, O, I;
int Bsearch(int idx, vector<int>& v)
{
int l = 0, r = v.size() - 1;
int result = -1;
while(l <= r)
{
int mid = (l + r) / 2;
if(v[mid] > idx) {
result = mid;
r = mid - 1;
}
else {
l = mid + 1;
}
}
return result;
}
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin >> n >> k >> s;
for(int i=0;i<s.size();i++)
{
if(s[i] == 'J')
J.push_back(i);
if(s[i] == 'O')
O.push_back(i);
if(s[i] == 'I')
I.push_back(i);
}
// for(int i=0;i<n;i++) cout << i; cout << '\n';
int res = INF;
for(int i = k-1; i < J.size(); i++)
{
int end_J = i;
int start_J = i - k + 1;
int start_O = Bsearch(J[end_J], O);
if(start_O == -1 || start_O + k - 1 >= O.size()) continue;
int end_O = start_O + k - 1;
int start_I = Bsearch(O[end_O], I);
if(start_I == -1 || start_I + k - 1 >= I.size()) continue;
int end_I = start_I + k - 1;
int sum = (I[end_I] - J[start_J] + 1) - (3 * k);
res = min(res, sum);
}
cout << (res == INF ? -1 : res);
return 0;
}
