/*
Author: Nguyen Chi Thanh - High School for the Gifted - VNU.HCM (i2528)
*/
#include <bits/stdc++.h>
using namespace std;
/* START OF TEMPALTE */
// #define int long long
#define ll long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define MASK(x) (1ll << (x))
#define SZ(a) ((int32_t)a.size())
#define debug(a, l, r) {for (int _i = (l); _i <= (r); ++_i) cout << (a)[_i] << ' '; cout << '\n';}
template<class X, class Y>
bool minimize(X &x, const Y &y) {
if (x > y) {
x = y;
return true;
} else return false;
}
template<class X, class Y>
bool maximize(X &x, const Y &y) {
if (x < y) {
x = y;
return true;
} else return false;
}
/* END OF TEMPALTE */
const int MAXN = 2e5 + 5;
int n, k, a[MAXN];
void init() {
string s;
cin >> n >> k >> s;
s = ' ' + s;
for (int i = 1; i <= n; ++i)
if (s[i] == 'J') a[i] = 0;
else if (s[i] == 'O') a[i] = 1;
else a[i] = 2;
}
int jump[3][MAXN], nxt[3][MAXN];
void build_jump(int x) {
for (int i = 1; i <= n; ++i) {
nxt[x][i] = n + 1;
jump[x][i] = n + 1;
}
vector<int> pos;
for (int i = 1; i <= n; ++i)
if (a[i] == x) pos.push_back(i);
for (int i = 0; i <= SZ(pos) - k; ++i)
jump[x][pos[i]] = pos[i + k - 1];
for (int i = n - 1; i >= 1; --i)
if (a[i + 1] == x) nxt[x][i] = i + 1;
else nxt[x][i] = nxt[x][i + 1];
}
void solve() {
for (int i = 0; i < 3; ++i)
build_jump(i);
int minlen = n + 5;
for (int i = 1; i <= n; ++i) {
if (a[i] != 0) continue;
int ptr = jump[0][i];
if (ptr > n) continue;
ptr = nxt[1][ptr];
if (ptr > n) continue;
ptr = jump[1][ptr];
if (ptr > n) continue;
ptr = nxt[2][ptr];
if (ptr > n) continue;
ptr = jump[2][ptr];
if (ptr > n) continue;
int len = ptr - i + 1;
minimize(minlen, len);
}
if (minlen > n) cout << -1;
else cout << minlen - 3 * k;
}
signed main() {
#ifdef NCTHANH
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
init();
solve();
return 0;
}
