//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#define int long long
typedef pair<int, int> T;
const int oo = 1e18, oo2 = 1e9+7, K = 30;
const int mod = 998244353;
int change(char c){
if (c == 'J') return 0;
if (c == 'O') return 1;
return 2;
}
void solve(){
int n, k; cin >> n >> k;
string s; cin >> s;
s = "$" + s;
vector nxt(n+1, vector<int>(3, -1));
for (int rep = 0; rep < 3; rep++){
int R = 1;
int cnt = (change(s[1]) == rep);
for (int L = 1; L <= n; L++){
while (R+1 <= n && cnt < k){
R++;
cnt += (change(s[R]) == rep);
}
if (cnt == k){
nxt[L][rep] = R;
}
cnt -= (change(s[L]) == rep);
}
}
for (int i = 1; i<=n; i++){
debug(i, nxt[i]);
}
int ans = oo;
for (int i = 1; i<=n; i++){
if (nxt[i][0] == -1) break;
int x = nxt[i][0]+1;
if (x > n || nxt[x][1] == -1) continue;
int y = nxt[x][1]+1;
if (y > n || nxt[y][2] == -1) continue;
int j = nxt[y][2];
debug(i, j);
ans = min(ans, (j-i+1));
}
if (ans == oo) cout << "-1\n";
else cout << ans - 3 * k << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) solve();
return 0;
}
