#include<bits/stdc++.h>
using namespace std;
bool M1;
#define PI 3.14159265358979323846
#define sz(a) (int)a.size()
#define all(x) x.begin(),x.end()
#define ii pair<int,int>
#define iii pair<int,ii>
#define iv pair<ii,ii>
#define se second
#define fi first
#define ffi fi.fi
#define sfi se.fi
#define sse se.se
#define fse fi.se
#define lt(i, c, d) for(int i = c; i <= d; ++i)
#define fl(i, c, d) for(int i = d; i >= c; --i)
#define pb push_back
#define emb emplace_back
#define emf emplace_front
#define em emplace
#define look_memory cerr<<abs(&M2-&M1)/1024.0/1024<<'\n'
#define look_time cerr << "TIME : " << clock() * 0.001 << "s" <<'\n'
const int N=1e6+5,lg=30,mod=1e9+7,inf=1e9;
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
int Rand(int u,int v){
return u+rd()%(v-u+1);
}
int dx[]={1,0,-1,0,1,1,-1,-1};
int dy[]={0,-1,0,1,1,-1,1,-1};
int node,k;
string s;
bool M2;
void solve(){
cin >> node >> k >> s;
s=" "+s;
int ans=inf;
int CI=0,CJ=0,CO=0;
int J=1,O=1,I=1;
for(int i=1;i<=node;++i){
CI+=(s[i]=='I');
while(CI-(s[I]=='I')>=k){
CI-=(s[I]=='I');
CO+=(s[I]=='O');
++I;
}
while(CO-(s[O]=='O')>=k){
CO-=(s[O]=='O');
CJ+=(s[O]=='J');
++O;
}
while(CJ-(s[J]=='J')>=k){
CJ-=(s[J]=='J');
++J;
}
if(CJ==k){
ans=min(ans,i-J+1-3*k);
}
}
if(ans==inf){
ans=-1;
}
cout << ans;
}
main()
{
srand(time(0));
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
#define task "aws"
if(fopen(task".inp","r")){
freopen(task".inp","r",stdin);
freopen(task".out","w",stdout);
}
int t=1;
// cin >> t;
while(t--){
solve();cout<<'\n';
}
look_memory;
look_time;
}
