#include <bits/extc++.h>
#define int long long
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define tam(x) (int)(x).size()
#define endl '\n'
#define sd second
#define fs first
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define optmizar_io ios_base::sync_with_stdio(0); cin.tie(0);
#define Setpre(n) cout << fixed <<setprecision(n)
#define dbg(x) for(auto y: x) cout << y <<' '; cout << endl;
#define dbg1(x) for(auto y: x) cout << y.first <<' '<< y.second << endl;
#define dbg2(v) for(auto x: v) cout << x.a<< ' '<< x.b <<' '<< x.c << endl;
const int MAX = 501;
const int INF = 1e17;
const int Mod = 1e9+7;
const int zero = 0;
const double eps = 1e-6;
const double PI=acos(-1.0);
const int mox[8] ={-1,1,0,0,-1,-1,1,1};
const int moy[8] ={0,0,-1,1,-1,1,-1,1};
using namespace std;
using namespace __gnu_pbds;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pi;
typedef vector<bool> vbol;
typedef array<int, 3> arr;
template <typename T>
using ordered_set = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;
void cases(string s){
#ifndef ONLINE_JUDGE
if(tam(s) == 0) s = "1";
string a = s + ".in";
string b = s + ".out";
freopen(a.c_str(), "r", stdin);
freopen(b.c_str(), "w", stdout);
#endif
}
void _1()
{
int n, k; cin >> n >> k;
string s; cin >> s;
vi j1, o1, i1;
for(int i = 0; i < n; i++)
{
if(s[i] == 'J') j1.pb(i);
else if(s[i] == 'O') o1.pb(i);
else i1.pb(i);
}
int ans = INF;
for(int i = 0; i + k <= tam(j1); i++)
{
auto os = lower_bound(all(o1), j1[i + k - 1] + 1);
if(os + k - 1 >= o1.end()) continue;
auto is = lower_bound(all(i1), o1[os - o1.begin() + k - 1] + 1);
if(is + k - 1 >= i1.end()) continue;
auto end = i1[is - i1.begin() + k - 1];
mins(ans, (end - j1[i] + 1) - 3 * k);
}
if(ans == INF) cout << -1 << endl;
else cout << ans << endl;
}
int32_t main() {
optmizar_io
// cases("");
//start:
//end:
//tags:
//difficulty:
//comment:
int T = 1; //cin >> T;
while(T--)
_1();
return 0;
}
