#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define rc(x) return cout<<x<<endl,0
#define pb push_back
#define mkp make_pair
#define in insert
#define er erase
#define fd find
#define fr first
#define sc second
typedef long long ll;
typedef long double ld;
const ll INF=0x3f3f3f3f3f3f3f3f;
const ll llinf=(1LL<<62);
const int inf=(1<<30);
const int nmax=2e5+50;
const int mod=1e9+7;
using namespace std;
int n,k,i,j,t,sz[3],rs;
char c;
vector<int>vc[3];
int get(char ch)
{
if(ch=='J')return 0;
else if(ch=='O')return 1;
else return 2;
}
int main()
{
//freopen("sol.in","r",stdin);
//freopen("sol.out","w",stdout);
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ios_base::sync_with_stdio(false);cin.tie(0);cerr.tie(0);cout.tie(0);
cin>>n>>k;
for(i=1;i<=n;i++)
{
cin>>c;
vc[get(c)].pb(i);
}
rs=inf;
for(i=0;i<3;i++)sz[i]=(int)vc[i].size();
for(i=0;i<sz[0];i++)
{
if(i+k<=sz[0])
{
j=lower_bound(vc[1].begin(),vc[1].end(),vc[0][i+k-1])-vc[1].begin();
if(j+k<=sz[1])
{
t=lower_bound(vc[2].begin(),vc[2].end(),vc[1][j+k-1])-vc[2].begin();
if(t+k<=sz[2])rs=min(rs,vc[2][t+k-1]-vc[0][i]+1);
else break;
}
else break;
}
else break;
}
if(rs==inf)cout<<-1<<endl;
else cout<<rs-3*k<<endl;
return 0;
}
