#include<bits/stdc++.h>
using namespace std;
#define ff first
#define ss second
#define pb push_back
const int N=3e5+5;
int sf[N][3];
int ps[N][3];
int cnt[3];
int dp[N];
char s[N];
int n,k;
int fun(int x, int l){
if(l>=n+1)return l;
int lo=l,hi=n;
while(hi>=lo){
int mid=(hi+lo)/2;
//cout<<mid<<" "<<ps[mid][x]-ps[l-1][x]<<endl;
if(ps[mid][x]-ps[l-1][x] >=k){
hi=mid-1;
}
else{
lo=mid+1;
}
}
return lo;
}
signed main(){
cin>>n>>k;
map<char,int> mp;
mp['J']=0;
mp['O']=1;
mp['I']=2;
for(int i=1;i<=n;i++){
cin>>s[i];
for(int j=0;j<3;j++)ps[i][j]=ps[i-1][j];
ps[i][mp[s[i]]]++;
}
int ans=1e9;
for(int i=1;i<=n;i++){
int l=fun(0,i);
l++;
l=fun(1,l);
l++;
l=fun(2,l);
if(l<=n){
ans=min(ans,(l-i+1-3*k));
}
}
if(ans==1e9)ans=-1;
cout<<ans;
}
