#include <bits/stdc++.h>
#define int long long
using namespace std;
const int INF = 1e18;
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(nullptr);
int n, k;
string s;
cin >> n >> k >> s;
map<char, vector<int>> pos;
for (int i = 0; i < n; ++i)
{
pos[s[i]].push_back(i);
}
int res = INF;
for (int i = 0; i + k - 1 < (int)pos['J'].size(); ++i)
{
// pos['J'][i] --> first J
// pos['J'][i + k - 1] --> last J
int first_o = lower_bound(pos['O'].begin(), pos['O'].end(), pos['J'][i + k - 1]) - pos['O'].begin();
if (first_o + k - 1 >= (int)pos['O'].size())
{
continue;
}
// pos['O'][o + k - 1] --> last o
int first_i = lower_bound(pos['I'].begin(), pos['I'].end(), pos['O'][first_o + k - 1]) - pos['I'].begin();
// pos['I'][first_i + k - 1] --> last I
if (first_i + k - 1 >= (int)pos['I'].size())
{
continue;
}
res = min(res, pos['I'][first_i + k - 1] - pos['J'][i] + 1 - 3 * k);
}
cout << (res == INF ? -1 : res) << '\n';
}
//
