#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define pf push_front
#define mp make_pair
#define fi first
#define se second
#define int long long
#define all(x) (x).begin(), (x).end()
typedef long double ld;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;
typedef vector<vector<ll>> vvll;
typedef vector<string> vs;
typedef vector<vector<string>> vvs;
typedef vector<char> vc;
typedef vector<vector<char>> vvc;
typedef map<int, int> mii;
typedef unordered_map<int, int> umii;
const int mod = 1e9 + 7;
const int inf = INTMAX_MAX;
const bool tc = false;
int bin(int i, int k, vi &y) {
if (i == y.size()) return i;
int c = (i ? y[i - 1] : (int)0);
int lo = i; int hi = y.size();
while (hi > lo + 1) {
int mid = (lo + hi) / 2;
if (y[mid] - c >= k) hi = mid;
else lo = mid;
}
return (hi == y.size() ? y.size() : hi);
}
inline void solve() {
int n, k;
cin >> n >> k;
string s; cin >> s;
vi jp(n), op(n), ip(n);
for (int i = 0; i < n; i++) {
if (s[i] == 'J') jp[i]++;
else if (s[i] == 'O') op[i]++;
else ip[i]++;
}
for (int i = 1; i < n; i++) {
jp[i] += jp[i - 1];
op[i] += op[i - 1];
ip[i] += ip[i - 1];
}
int ans = 1e18;
for (int i = 0; i < n; i++) {
if (s[i] != 'J') continue;
int jc = 0, oc = 0, ic = 0;
int ptr = bin(i, k, jp); ptr = bin(ptr, k, op); ptr = bin(ptr, k, ip);
if (ptr != n) ans = min(ans, ptr - i - (3 * k - 1));
}
cout << (ans == 1e18 ? -1 : ans) << '\n';
}
void setIO(string s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
signed main() {
ios::sync_with_stdio(false);
cout.tie(0);
cin.tie(0);
//setIO();
int t = 1;
if (tc) {
cin >> t;
}
while (t--) {
solve();
}
}
