#include <bits/stdc++.h>
#define f first
#define s second
#define int long long
using namespace std;
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, k;
string s;
cin >> n >> k >> s;
vector<int> jj, oo, ii;
for (int i = 0; i < n; i++) {
if(s[i] == 'J')
jj.push_back(i);
else if(s[i] == 'O')
oo.push_back(i);
else
ii.push_back(i);
}
int ans = 1e9;
for (int i = 0;i + k - 1 < (int)jj.size();i++) {
auto u = upper_bound(oo.begin(), oo.end(), jj[i + k - 1]) - oo.begin();
if(u + k - 1 >= (int)oo.size())
continue;
auto l = upper_bound(ii.begin(), ii.end(), oo[u + k - 1]) - ii.begin();
if(l + k - 1 >= (int)ii.size())
continue;
int t = n - jj[i] - (n - 1 - ii[l + k - 1]);
ans = min(ans, t - (3 * k));
}
if(ans == 1e9)
ans = -1;
cout << ans;
return 0;
}
