#include <bits/stdc++.h>
#define int long long
int give_sum(int l, int r, std::vector<int> &pref)
{
if (l > r)
{
std::swap(l, r);
}
if (l == 0)
{
return pref[r];
}
return pref[r] - pref[l - 1];
}
signed main()
{
std::ios::sync_with_stdio(false);
std::cin.tie(0);
int n, k;
std::cin >> n >> k;
std::string s;
std::cin >> s;
std::vector<int> prev_j(n);
std::vector<int> next_i(n);
std::vector<int> pref_j(n);
std::vector<int> pref_i(n);
std::vector<int> pref_o(n);
pref_j[0] = (s[0] == 'J');
pref_i[0] = (s[0] == 'I');
pref_o[0] = (s[0] == 'O');
int last_j = -1;
for (int i = 0; i < n; i++)
{
if (s[i] == 'J')
{
last_j = i;
}
prev_j[i] = last_j;
pref_j[i] = pref_j[i - 1] + (s[i] == 'J');
pref_i[i] = pref_i[i - 1] + (s[i] == 'I');
pref_o[i] = pref_o[i - 1] + (s[i] == 'O');
}
int last_i = -1;
for (int i = n - 1; i >= 0; i--)
{
if (s[i] == 'I')
{
last_i = i;
}
next_i[i] = last_i;
}
int ans = 1e9;
for (int i = 0; i < n; i++)
{
if (s[i] != 'O')
continue;
int l = i - 1, r = n;
while (l + 1 < r)
{
int mid = (l + r) / 2;
if (give_sum(i, mid, pref_o) < k)
{
l = mid;
}
else
{
r = mid;
}
}
if (r == n)
{
continue;
}
int start = i;
int stop = r;
int now = (stop - start + 1) - k;
int tmp = prev_j[i];
if (tmp == -1)
{
continue;
}
now += (start - tmp - 1);
l = -1, r = tmp;
while (l + 1 < r)
{
int mid = (l + r) / 2;
if (give_sum(mid, tmp, pref_j) < k)
{
r = mid;
}
else
{
l = mid;
}
}
if (l == -1)
{
continue;
}
now += (std::abs(tmp - l) + 1) - k;
tmp = next_i[stop];
if (tmp == -1)
{
continue;
}
now += (std::abs(stop - tmp) - 1);
l = tmp - 1,
r = n;
while (l + 1 < r)
{
int mid = (l + r) / 2;
if (give_sum(tmp, mid, pref_i) < k)
{
l = mid;
}
else
{
r = mid;
}
}
if (r == n)
{
continue;
}
// std::cout << now << ' ' << r << ' ' << tmp << std::endl;
now += (r - tmp + 1) - k;
// std::cout << now << std::endl;
ans = std::min(ans, now);
}
if (ans == 1e9)
{
std::cout << -1;
return 0;
}
std::cout << ans;
}
