#include <bits/stdc++.h>
#pragma GCC optimize ("Ofast", "unroll-loops")
using namespace std;
#define ll long long
#define int ll
#define FOR(i,a,b) for (int i = (a); i<(b); i++)
#define REP(i,n) FOR(i,0,n)
#define REP1(i,n) FOR(i,1,n+1)
#define RREP(i,n) for (int i=(n)-1; i>=0; i--)
#define RREP1(i,n) for (int i=(n); i>=1; i--)
#define f first
#define s second
#define pb push_back
#define ALL(x) x.begin(),x.end()
#define SZ(x) (int)(x.size())
#define SQ(x) (x)*(x)
#define pii pair<int, int>
#define pdd pair<double ,double>
#define pcc pair<char, char>
#define endl '\n'
//#define TOAD
#ifdef TOAD
#define bug(x) cerr<<__LINE__<<": "<<#x<<" is "<<x<<endl
#define IOS()
#else
#define bug(...)
#define IOS() ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#endif
const ll inf = 1ll<<60;
const int iinf=2147483647;
const ll mod = 1e9+7;
const ll maxn=5005;
const double PI=acos(-1);
ll pw(ll x, ll p, ll m=mod){
ll ret=1;
while (p>0){
if (p&1){
ret*=x;
ret%=m;
}
x*=x;
x%=m;
p>>=1;
}
return ret;
}
ll inv(ll a, ll m=mod){
return pw(a,m-2);
}
//=======================================================================================
vector<int> pf[3];
signed main (){
IOS();
int n, k; cin>>n>>k;
REP(i, 3) pf[i].resize(n);
string str; cin>>str;
vector<int> pfj(n), pfo(n), pfi(n);
REP(i,n){
pf[0][i]=(str[i]=='J');
pf[1][i]=(str[i]=='O');
pf[2][i]=(str[i]=='I');
if (i>0){
REP(j, 3){
pf[j][i]+=pf[j][i-1];
}
}
}
vector<char> mat(3);
mat[0]='J'; mat[1]='O'; mat[2]='I';
int ans=inf;
REP(i,n){
if (str[i]!='J') continue;
int a=i;
bool gg=false;
REP(j, 3){
if (a>=n){
gg=1;
break;
}
//cout<<a<<' '<<str[a]<<endl;
a=lower_bound(ALL(pf[j]), pf[j][a]+k-(str[a]==mat[j]))-pf[j].begin();
if (a>=n) {
gg=1;
break;
}
a++;
}
if (!gg) ans=min(ans, a-i-k*3);
}
if (ans==inf) ans=-1;
cout<<ans<<endl;
}
