#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pii pair<int,int>
#define sz(x) (int)(x.size())
#define all(x) x.begin(),x.end()
#define F first
#define S second
#define pb push_back
#ifdef local
void setio(){freopen("/Users/iantsai/Library/Mobile Documents/com~apple~CloudDocs/cpp/Empty.md","r",stdin);}
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
void setio(){}
#define debug(x...)
#endif
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
setio();
int n,k;
cin>>n>>k;
string s;
cin>>s;
s=' '+s;
vector<vector<int>>pos(3);
for(int i=1;i<=n;i++){
if(s[i]=='J')pos[0].pb(200000-i);
else if(s[i]=='O')pos[1].pb(i);
else if(s[i]=='I')pos[2].pb(i);
}
reverse(all(pos[0]));
int ans=8e18;
//for(auto i:pos[0])cout<<i<<' ';
//cout<<'\n';
for(int i=0;i+k<=sz(pos[1]);i++){
int l=pos[1][i];
int r=pos[1][i+k-1];
int temp=r-l+1-k;
l=200000-l;
//cout<<temp<<' ';
int x=lower_bound(all(pos[0]),l)-pos[0].begin();
if(x+k>sz(pos[0]))continue;
int y=lower_bound(all(pos[2]),r)-pos[2].begin();
if(y+k>sz(pos[2]))continue;
//cout<<pos[0][x+k-1]<<' '<<l<<' ';
temp+=pos[0][x+k-1]-l-k+pos[2][y+k-1]-r-k;
ans=min(ans,temp);
}
if(ans==8e18){
cout<<-1<<'\n';
}
else{
cout<<ans<<'\n';
}
}
