#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
string s;
cin >> n >> k >> s;
auto get_val = [](char& c) {
if(c == 'J') return 0;
if(c == 'O') return 1;
return 2;
};
vector pref(3, vector<int>(n + 1));
for(int i = 0; i < n; ++i) {
for(int c = 0; c < 3; ++c) {
pref[c][i + 1] = pref[c][i];
}
pref[get_val(s[i])][i + 1]++;
}
if(min({pref[0][n], pref[1][n], pref[2][n]}) < k) {
cout << -1;
return 0;
}
auto chk = [&](int c, int l, int r) {
return pref[c][r + 1] - pref[c][l];
};
auto nxt = [&](int c, int l, int need) {
int lo = l, hi = n - 1, ans = n;
while(lo <= hi) {
int mid = (lo + hi) >> 1;
if(chk(c, l, mid) >= need) {
hi = mid - 1;
ans = mid;
} else {
lo = mid + 1;
}
}
return ans;
};
const int inf = 1 << 30;
int ans = inf;
for(int l = 0; l < n; ++l) {
if(s[l] != 'J') continue;
int en = nxt(0, l, k);
if(en == n) continue;
int en2 = nxt(1, en + 1, k);
if(en2 == n) continue;
int en3 = nxt(2, en2 + 1, k);
if(en3 == n) continue;
ans = min(ans, en3 - l + 1 - 3 * k);
}
cout << (ans == inf ? -1 : ans);
return 0;
}
