#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define fi first
#define se second
constexpr ll MAXN=1e6+5,MAXV=3e5,MOD=1e9+7,INF=1e18;
ll n,m,i,j,p,k,ans,dem,pre[MAXN],suf[MAXN];
string s;
ll dist(ll st,ll en){
return en-st+1-k;
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
cin>>n>>k>>s;
ll cntO=0,cntJ=0,cntI=0;
queue<ll> O,I,J;
pre[0]=suf[n+1]=-1;
for(i=1;i<=n;i++){
if(s[i-1]=='J') J.push(i);
while(J.size()>k) J.pop();
if(J.size()==k)
pre[i]=J.front();
else pre[i]=-1;
}
for(i=n;i>=1;i--){
if(s[i-1]=='I') I.push(i);
while(I.size()>k) I.pop();
if(I.size()==k)
suf[i]=I.front();
else suf[i]=-1;
}
ll ans=INF;
for(i=1;i<=n;i++){
if(s[i-1]=='O') O.push(i);
while(O.size()>k) O.pop();
if(O.size()==k){
ll st=O.front();
if(suf[i+1]==-1||pre[st-1]==-1) continue;
ll p=pre[st-1],s=suf[i+1];
ans=min(ans,dist(p,st-1)+dist(st,i)+dist(i+1,s));
}
}
if(ans==INF){
cout<<-1;
return 0;
}
cout<<ans;
}
