#include <bits/stdc++.h>
//#define int long long
//#pragma GCC optimize("Ofast")
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4")
#define file(s) freopen(s".in","r",stdin); freopen(s".out","w",stdout);
#define forev(i, b, a) for(int i = (b); i >= (a); --i)
#define forn(i, a, b) for(int i = (a); i <= (b); ++i)
#define all(x) x.begin(), x.end()
#define sz(s) (int)s.size()
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define s second
#define f first
using namespace std;
typedef pair < long long, long long > pll;
typedef pair < int, int > pii;
typedef unsigned long long ull;
typedef vector < pii > vpii;
typedef vector < int > vi;
typedef long double ldb;
typedef long long ll;
typedef double db;
const int dx[] = {1, -1, 0, 0}, dy[] = {0, 0, 1, -1}, block = 555;
const pii base = mp(1171, 3307), Mod = mp(1e9 + 7, 1e9 + 9);
const int inf = 1e9, maxn = 4e5 + 148, mod = 1e9 + 7, N = 2e5 + 11;
const db eps = 1e-12, pi = 3.14159265359;
const ll INF = 1e18;
int n, k, a[N];
string s;
vi g[N];
main () {
cin >> n >> k >> s;
forn (i, 0, n - 1)
if (s[i] == 'J')
g[0].pb(i);
else if (s[i] == 'O')
g[1].pb(i);
else
g[2].pb(i);
if (sz(g[0]) < k || sz(g[1]) < k || sz(g[2]) < k) {
cout << -1;
return 0;
}
int ans = inf;
forn (i, 0, sz(g[0]) - k) {
int j = lower_bound(all(g[1]), g[0][i + k - 1]) - g[1].begin();
if (j + k - 1 >= sz(g[1]))
break;
int x = lower_bound(all(g[2]), g[1][j + k - 1]) - g[2].begin();
if (x + k - 1 >= sz(g[2]))
break;
ans = min(ans, g[2][x + k - 1] - g[0][i] + 1 - 3*k);
}
if (ans == inf)
ans = -1;
cout << ans << endl;
return 0;
}
