#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O2,no-stack-protector,unroll-loops,fast-math")
#define F first
#define S second
#define pb push_back
#define SZ(x) (ll)(x.size())
#define all(x) x.begin(),x.end()
#define MP make_pair
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const ll maxn=2e5+10, maxm=1e3+10, lg=21, mod=1e9+7, inf=1e18;
ll n,k,ans=inf;
string s;
vector<ll> vv[3];
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
cin>>n>>k>>s;
for(int i=0;i<n;i++) vv[(s[i]=='J' ? 0:(s[i]=='O' ? 1:2))].pb(i);
ll i[3]={0,0,0};
for(;i[0]<SZ(vv[0]);i[0]++){
bool F=0;
for(int j:{1,2}){
if(i[j-1]+k-1>=SZ(vv[j-1])){F=1;break;}
while(i[j]<SZ(vv[j]) && vv[j][i[j]]<vv[j-1][i[j-1]+k-1]) i[j]++;
}
if(i[2]+k-1>=SZ(vv[2])) F=1;
if(F) break;
//cout<<i[0]<<' '<<i[1]<<' '<<i[2]<<endl;
ans=min(ans,vv[2][i[2]+k-1]-vv[0][i[0]]+1-3*k);
}
cout<<(ans==inf ? -1:ans);
return 0;
}
