/*input
9 1
IIIOOOJJJ
*/
//#include "supertrees.h"
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
//order_of_key #of elements less than x
// find_by_order kth element
typedef long long int ll;
#define ld double
#define pii pair<int,int>
#define f first
#define s second
#define pb push_back
#define REP(i,n) for(ll i=0;i<n;i++)
#define REP1(i,n) for(int i=1;i<=n;i++)
#define FILL(n,x) memset(n,x,sizeof(n))
#define ALL(_a) _a.begin(),_a.end()
#define sz(x) (int)x.size()
const ll maxn=2e5+5;
const ll maxlg=__lg(maxn)+2;
const ll INF64=4e18;
const int INF=0x3f3f3f3f;
const ll MOD=(1e9+7);
const ll MOD2=1000002173;
const ld PI=acos(-1);
const ld eps=1e-9;
#define lowb(x) x&(-x)
#define MNTO(x,y) x=min(x,(__typeof__(x))y)
#define MXTO(x,y) x=max(x,(__typeof__(x))y)
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),x)-c.begin())
ll mult(ll a,ll b){
return ((a%MOD)*(b%MOD))%MOD;
}
ll mypow(ll a,ll b,ll MOD){
if(b<=0) return 1;
ll res=1LL;
while(b){
if(b&1) res=(res*a)%MOD;
a=(a*a)%MOD;
b>>=1;
}
return res;
}
vector<int> v[3];
int main(){
ios::sync_with_stdio(false),cin.tie(0);
int n,k;
cin>>n>>k;
REP(i,n){
char x;
cin>>x;
if(x=='J') v[0].pb(i);
else if(x=='O') v[1].pb(i);
else v[2].pb(i);
}
int ans=INF;
REP(i,sz(v[0])-k+1){
int p=v[0][i+k-1];
int nx=GET_POS(v[1],p)+k-1;
if(nx>=sz(v[1])){
break;
}
p=v[1][nx];
nx=GET_POS(v[2],p)+k-1;
if(nx>=sz(v[2])){
break;
}
MNTO(ans,v[2][nx]-v[0][i]+1-3*k);
}
if(ans==INF) ans=-1;
cout<<ans;
}
