#include <bits/stdc++.h>
using namespace std;
#define fastio \
ios::sync_with_stdio(false); \
cin.tie(nullptr);
#define s(x) x.size()
#define sort(x) sort(x.begin(), x.end())
#define rsort(x) sort(x.rbegin(), x.rend())
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define fi first
#define se second
#define INF 1e9
#define INFL 1e18
#define rep(a, b) for (int a = 0; a < b; a++)
using ll = long long;
using pii = pair<int, int>;
void solve()
{
int n, k;
string s;
cin >> n >> k >> s;
int l = 0, p = 0;
vector<int> dpj(n, -1);
int suma = 0;
while (p < n)
{
if (s[p] == 'J')
suma++;
while (suma > k || s[l] != 'J')
{
if (s[l] == 'J')
suma--;
l++;
}
if (suma == k)
dpj[p] = p - l + 1 - k;
p++;
}
l = 0, p = 0;
vector<pair<int, int>> dpo(n, {-1, -1});
suma = 0;
while (p < n)
{
if (s[p] == 'O')
suma++;
while (suma > k || s[l] != 'O')
{
if (s[l] == 'O')
suma--;
l++;
}
if (suma == k)
dpo[p] = {l, p - l + 1 - k};
p++;
}
l = n - 1, p = n - 1;
vector<int> dpi(n, -1);
suma = 0;
while (p >= 0)
{
if (s[p] == 'I')
suma++;
while (suma > k || s[l] != 'I')
{
if (s[l] == 'I')
suma--;
l--;
}
if (suma == k)
dpi[p] = l - p + 1 - k;
p--;
}
int minn = 1e9;
// for (int i = 0; i < n; i++)
// {
// cout << dpj[i] << ' ';
// }
// cout << endl;
// for (int i = 0; i < n; i++)
// {
// cout << dpo[i].second << ' ';
// }
// cout << endl;
// for (int i = 0; i < n; i++)
// {
// cout << dpi[i] << ' ';
// }
// cout << endl;
for (int i = 1; i < n - 1; i++)
{
if (dpo[i].first > 0)
if (dpj[dpo[i].first-1] != -1 && dpo[i].first != -1 && dpi[i + 1] != -1)
minn = min(minn, dpj[dpo[i].first-1] + dpo[i].second + dpi[i + 1]);
}
if (minn != 1e9)
cout << minn << endl;
else
cout << -1 << endl;
}
int main()
{
fastio;
int t = 1;
// cin >> t;
while (t--)
solve();
return 0;
}
