#include<bits/stdc++.h>
#define int long long
using namespace std;
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n,k;
cin >> n >> k;
string a,s;
cin >> s;
for(int i=0;i<k;i++) a+='J';
for(int i=0;i<k;i++) a+='O';
for(int i=0;i<k;i++) a+='I';
int cnt = 0,ok = 0;
char cur = 'I';
int l = n-1,r = -1;
queue<int> j,o,i;
for(;l>=0;l--)
{
if(s[l]==cur)
{
if(cur=='J') j.push(l);
else if(cur=='I') i.push(l);
else o.push(l);
cnt++;
if(cnt==k)
{
if(cur=='J')
{
ok = 1;
break;
}
else if(cur=='O') cur = 'J';
else cur = 'O';
cnt = 0;
}
}
if(r==-1 && s[l]=='I')
{
r = l;
}
}
int ans = r-l+1;
if(ok==0)
{
cout << -1 << endl;
return 0;
}
l--;
for(;l>=0;l--)
{
if(s[l]!='J') continue;
j.push(l);
int ex = j.front();
j.pop();
int nw = j.front();
for(int konum = ex;konum>nw;konum--)
{
if(s[konum]=='O')
{
o.push(konum);
}
}
while(o.size()>k)
{
int ex = o.front();
o.pop();
int nw = o.front();
for(int konum = ex;konum>nw;konum--)
{
if(s[konum]=='I')
{
i.push(konum);
}
}
}
while(i.size()>k)
{
i.pop();
}
r = i.front();
// cout << l << " " << r << endl;
ans = min(ans,(i.front()-l+1));
}
cout << ans-(3*k) << endl;
}
