#include <bits/stdc++.h>
using namespace std;
#define name "ojuz"
#define MAXN 200005
#define pb push_back
#define pf push_front
#define ll long long
#define ii pair<int, int>
#define fs first
#define sc second
#define ill pair<int, ll>
#define lli pair<ll, int>
#define llll pair<ll, ll>
#define all(v) v.begin(),v.end()
#define uni(v) v.erase(unique(all(v)),v.end())
#define bit(n,i) (((n)>>(i))&1)
#define FOR(i,a,b) for (int i=(a),_b=(b); i<=_b; i++)
#define FORD(i,a,b) for (int i=(a),_b=(b); i>=_b; i--)
#define MASK(i) (1LL<<(i))
const int INF=1e9;
const int MOD=1e9+7;
void add(int &u, int v){
u+=v;
if (u>=MOD) u-=MOD;
}
void sub(int &u, int v){
u-=v;
if (u<0) u+=MOD;
}
void minimize(int &u, int v){
u=min(u,v);
}
void maximize(int &u, int v){
u=max(u,v);
}
long long Rand(long long l, long long r){
ll tmp=0;
FOR(i,1,4) tmp=((tmp<<15)^(((1<<15)-1)&rand()));
return l+tmp%(r-l+1);
}
int n,k;
char s[MAXN];
int a[MAXN];
int cnt[3][MAXN];
int get(int c, int l, int r){
return cnt[c][r]-cnt[c][l-1];
}
int main()
{
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
cin>>n>>k;
FOR(i,1,n) cin>>s[i];
FOR(i,1,n){
if (s[i]=='J') a[i]=0;
else if (s[i]=='O') a[i]=1;
else a[i]=2;
}
FOR(i,1,n){
FOR(j,0,2) cnt[j][i]=cnt[j][i-1];
cnt[a[i]][i]++;
}
int ans=INF;
int j=1;
FOR(i,1,n){
if (cnt[1][i]<k) continue;
while (cnt[1][i]-cnt[1][j]>=k) ++j;
int res_A=-1,res_B=-1;
int l=1,r=j-1;
while (l<=r){
int mid=(l+r)/2;
if (get(0,mid,j-1)>=k){
l=mid+1;
res_A=mid;
}
else r=mid-1;
}
l=i+1,r=n;
while (l<=r){
int mid=(l+r)/2;
if (get(2,i+1,mid)>=k){
r=mid-1;
res_B=mid;
}
else l=mid+1;
}
if (res_A==-1||res_B==-1) continue;
int len=res_B-res_A+1-3*k;
minimize(ans,len);
}
cout<<(ans==INF?-1:ans);
return 0;
}
