#include <bits/stdc++.h>
using namespace std;
int n,k; string s;
int jump(int i, vector<int> &v) {
if (i == -1 || v.empty() || v.back() < i)
return -1;
int j = (lower_bound(v.begin(),v.end(),i)-v.begin());
if (j+k-1 >= v.size())
return -1;
return v[j+k-1];
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> k >> s;
vector<int> indj, indo, indi;
for (int i = 0; i < n; i++) {
if (s[i] == 'J')
indj.push_back(i);
else if (s[i] == 'O')
indo.push_back(i);
else
indi.push_back(i);
}
int odp = 1000000000;
for (int i = 0; i < n; i++) {
int j = jump(jump(jump(i,indj),indo),indi);
if (j != -1)
odp = min(odp,j-i-3*k+1);
}
cout << ((odp == 1000000000) ? -1 : odp) << endl;
return 0;
}
