#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pb push_back
#define debug(x) cerr<<#x<<"="<<x<<endl
#define pq priority_queue
#define inf 1ll<<60
#define rep(i,a,b) for (ll i=a;i<(b);i++)
#define MP make_pair
#define SZ(x) (int(x.size()))
#define mod 1000000007
#define ALL(x) x.begin(),x.end()
#define endl "\n"
void inc(ll &a,ll b) {a=(a+b)%mod;}
void dec(ll &a,ll b) {a=(a-b+mod)%mod;}
int prod(ll a,ll b) {return ll(a)*ll(b)%mod;}
int lowbit(ll x) {return x&(-x);}
ll p0w(ll base,ll p) {ll ret=1;while(p>0){if (p%2ll==1ll) ret=ret*base%mod;base=base*base%mod;p/=2ll;}return ret;}
const ll maxn=222222;
ll f[3][maxn];
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);
ll n,k;cin>>n>>k;
string s;cin>>s;
s='0'+s;
rep(i,1,n+1){
f[0][i]=f[0][i-1];
f[1][i]=f[1][i-1];
f[2][i]=f[2][i-1];
if(s[i]=='J')f[0][i]++;
if(s[i]=='O')f[1][i]++;
if(s[i]=='I')f[2][i]++;
}
ll ans=inf;
rep(i,1,n+1){
ll a=lower_bound(f[0]+i,f[0]+n+1,f[0][i-1]+k)-f[0];
if(a>n)break;
ll b=lower_bound(f[1]+a,f[1]+n+1,f[1][a-1]+k)-f[1];
if(b>n)break;
ll c=lower_bound(f[2]+b,f[2]+n+1,f[2][b-1]+k)-f[2];
if(c>n)break;
ans=min(ans,c-i+1-3*k);
//find shortest substring with k of j,o and i in order
}
if(ans==inf)cout<<-1<<endl;
else cout<<ans<<endl;
return 0;
}
/*
*/
