#include <bits/stdc++.h>
#define NMAX 500001
#define pb push_back
#define MOD 1000000007
#define nl '\n'
#define INF 0x3f3f3f3f
#define pii pair<int,int>
using namespace std;
ifstream fin("aib.in");
ofstream fout("aib.out");
int n,k,ans=INF;
string s;
int vj[NMAX],vo[NMAX],vi[NMAX],li,lo,lj;
int J='J'-'0';
int O='O'-'0';
int I='I'-'0';
signed main() {
cin>>n>>k;
cin>>s;
for(int i=0;i<n;++i)
{
if(s[i]=='I')
vi[++li]=i;
if(s[i]=='J')
vj[++lj]=i;
if(s[i]=='O')
vo[++lo]=i;
// cout<<s[i];
}
int pj=1,po=1,pi=1;
while(pj+k-1<=lj&& po+k-1<=lo&& pi+k-1<=li)
{
while(po+k-1<=lo&& vo[po]<=vj[pj+k-1])
po++;
if(po+k-1<=lo)
while(pi+k-1<=li && vi[pi]<=vo[po+k-1])
pi++;
if(pj+k-1<=lj && po+k-1<=lo && pi+k-1<=li)
{
ans=min(ans,(vi[pi+k-1]-vj[pj]+1)-3*k);
}
pj++;
}
if(ans==INF)
cout<<-1;
else
cout<<ans;
return 0;
}
