#include <bits/stdc++.h>
#define int long long
#define debug cout << "ok\n";
#define SQR(x) (1LL * ((x) * (x)))
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define pii pair<int,int>
#define pli pair<ll,int>
#define vi vector<int>
#define FAST ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef unsigned int ui;
using namespace std;
const int M = 1e9 + 7;
const int INF = 1e9 + 7;
const ll INFLL = (ll)2e18 + 7LL;
const ld PI = acos(-1);
const int dx[] = {1, -1, 0, 0, -1, 1, 1, -1};
const int dy[] = {0, 0, 1, -1, -1, -1, 1, 1};
template<class _, class __>
bool minimize(_ &x, const __ y){
if(x > y){
x = y;
return true;
} else return false;
}
template<class _, class __>
bool maximize(_ &x, const __ y){
if(x < y){
x = y;
return true;
} else return false;
}
template<class _,class __>
void Add(_ &x, const __ y) {
x += y;
if (x >= M) {
x -= M;
}
return;
}
template<class _,class __>
void Diff(_ &x, const __ y) {
x -= y;
if (x < 0) {
x += M;
}
return;
}
//--------------------------------------------------------------
const int MaxN = 1e6+7;
int n,K,nxt[MaxN][3],res = INF,a[MaxN];
void sol() {
cin >> n >> K;
for (int i=1;i<=n;i++) {
char x; cin >> x;
if (x == 'J') a[i] = 0;
else if (x == 'O') a[i] = 1;
else a[i] = 2;
}
for (int i=0;i<3;i++) {
int cnt = 0;
for (int j=n,k=n+1;j>=1;j--) {
cnt += a[j] == i;
while (cnt >= K) {
k--;
cnt -= a[k] == i;
}
nxt[j][i] = k;
}
}
for (int i=1;i<=n;i++) {
if (nxt[i][0] < n) {
if (nxt[nxt[i][0]+1][1] < n) {
if (nxt[nxt[nxt[i][0]+1][1]+1][2] <= n) {
minimize(res,nxt[nxt[nxt[i][0]+1][1]+1][2] - i + 1 - 3 * K);
}
}
}
}
if (res < INF) cout << res;
else cout << -1;
}
signed main() {
// freopen("test.inp","r",stdin);
// freopen("test.out","w",stdout);
FAST
int t=1;
// cin >> t;
while (t--) sol();
}
