//~ #pragma GCC optimize("Ofast,unroll-loops")
//~ #pragma GCC target("avx,avx2,fma,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#define fast ios_base::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL)
#define fi first
#define se second
#define space " "
#define endl "\n"
#define mp make_pair
#define pb push_back
#define pf push_front
#define lb lower_bound
#define ub upper_bound
#define md 1000000007
#define inf 1000000000
#define li 500005
#define lo long long
using namespace std;
int T,Q,n,m,k,a[li],mn=inf;
string s,t;
vector<int> v[10];
map<char,int> harf;
vector<pair<int,int> > vv[10];
int main(){
fast;
cin>>n>>k>>s;
harf['J']=1;
harf['O']=2;
harf['I']=3;
for(int i=0;i<n;i++){
int son=i+1;
while(son<n && s[son]==s[i]) son++;
son--;
v[harf[s[i]]].pb(i);
vv[harf[s[i]]].pb(mp(i,son));
i=son;
}
//~ for(int i=1;i<=3;i++){
//~ for(int j=0;j<(int)v[i].size();j++){
//~ printf("NICE %d %d %d\n",i,j,v[i][j]);
//~ }
//~ }
int sum=0,son=0;
for(int i=0;i<(int)vv[2].size();i++){
while(son<(int)vv[2].size() && sum<k){
sum+=vv[2][son].se-vv[2][son].fi+1;
son++;
}
if(sum<k) break;
//~ printf("DEBUG %d\n",son);
//~ printf("ABUBA %d\n",vv[2][i].fi);
auto itr=upper_bound(v[1].begin(),v[1].end(),vv[2][i].fi);
if(itr==v[1].begin()){sum-=vv[2][i].se-vv[2][i].fi+1; continue;}
itr--;
int ind1=itr-v[1].begin();
//~ printf("ABUBEEE %d\n",ind1);
auto itr2=upper_bound(v[3].begin(),v[3].end(),vv[2][son-1].se);
if(itr2==v[3].end()){sum-=vv[2][i].se-vv[2][i].fi+1; continue;}
int ind2=itr2-v[3].begin();
//~ printf("ABUBEEE %d %d\n",ind1,ind2);
int cev=vv[3][ind2].fi-vv[1][ind1].se-1-k;
int sayi1=0;
for(int j=ind1;j>=0;j--){
sayi1+=vv[1][j].se-vv[1][j].fi+1;
if(j!=ind1) cev+=vv[1][j+1].fi-vv[1][j].se-1;
if(sayi1>=k) break;
}
int sayi2=0;
for(int j=ind2;j<(int)vv[3].size();j++){
sayi2+=vv[3][j].se-vv[3][j].fi+1;
if(j!=ind2) cev+=vv[3][j].fi-vv[3][j-1].se-1;
if(sayi2>=k) break;
}
if(sayi1<k || sayi2<k){sum-=vv[2][i].se-vv[2][i].fi+1; continue;}
mn=min(mn,cev);
sum-=vv[2][i].se-vv[2][i].fi+1;
}
if(mn==inf) cout<<"-1\n";
else cout<<mn<<endl;
return 0;
}
