//#pragma GCC optimize("O3")
#include<bits/stdc++.h>
using namespace std;
#define int long long
#define X first
#define Y second
int n;int K;
string s;
int psj[200005];
int pso[200005];
int psi[200005];
int dpj[200005];
int dpi[200005];
signed main(){
cin>>n>>K>>s;
s="*"+s;
for(int i=1;i<=n;i++){
psj[i]=psj[i-1];
pso[i]=pso[i-1];
psi[i]=psi[i-1];
if(s[i]=='I'){
psi[i]++;
}
if(s[i]=='O'){
pso[i]++;
}
if(s[i]=='J'){
psj[i]++;
}
}
for(int i=1;i<=n;i++){
dpj[i]=1e16;
if(s[i]!='J'){continue;}
if(psj[i]<K){continue;}
int l=0;int r=i;
while(l<r){
int mi=l+(r-l+1)/2;
if(mi==0||(mi!=0&&psj[i]-psj[mi-1]>=K)){
l=mi;
}else{
r=mi-1;
}
}
if(l!=0){
dpj[i]=i-l+1-K;
}
}
for(int i=1;i<=n;i++){
dpi[i]=1e16;
if(s[i]!='I'){continue;}
if(psi[n]-psi[i-1]<K){continue;}
int l=i+K-1;int r=n+1;
while(l<r){
int mi=l+(r-l)/2;
if(mi==n+1||(mi!=n+1&&psi[mi]-psi[i-1]>=K)){
r=mi;
}else{
l=mi+1;
}
}
if(l!=n+1){
dpi[i]=l-i+1-K;
}
}
dpj[0]=1e16;
int ans=1e16;
int mnv=1e16;
int lit=0;
for(int i=1;i<=n;i++){
if(dpi[i]<=n&&pso[i-1]>=K){
while(lit+1<=n&&pso[i-1]-pso[lit+1]>=K){
lit++;
mnv=min(mnv,dpj[lit]-lit);
}
ans=min(ans,dpi[i]+i-1+mnv-K);
}
}
/*
for(int i=1;i<=n;i++){
if(dpj[i]>n){dpj[i]=-1;}
if(dpi[i]>n){dpi[i]=-1;}
cout<<dpj[i]<<" ";
}cout<<endl;
for(int i=1;i<=n;i++){
cout<<dpi[i]<<" ";
}*/
if(ans>n){
cout<<-1<<endl;return 0;
}
cout<<ans<<endl;
}
