#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define fr first
#define sc second
int n, k;
int pref1[200001], pref2[200001], pref3[200001];
int bin_search1(int val)
{
int l = 1, r = n;
while(l<=r)
{
int mid = ((l+r)>>1);
if (pref1[mid] < val)
{
l = mid+1;
}
else
{
r = mid - 1;
if (pref1[mid] < val)
{
return mid;
}
}
}
if (l>n)
{
return -1;
}
return l;
}
int bin_search2(int val)
{
int l = 1, r = n;
while(l<=r)
{
int mid = ((l+r)>>1);
if (pref2[mid] < val)
{
l = mid+1;
}
else
{
r = mid - 1;
if (pref2[mid] < val)
{
return mid;
}
}
}
if (l>n)
{
return -1;
}
return l;
}
int bin_search3(int val)
{
int l = 1, r = n;
while(l<=r)
{
int mid = ((l+r)>>1);
if (pref3[mid] < val)
{
l = mid+1;
}
else
{
r = mid - 1;
if (pref3[mid] < val)
{
return mid;
}
}
}
if (l>n)
{
return -1;
}
return l;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cin>>n>>k;
pref1[0] = 0;
pref2[0] = 0;
pref3[0] = 0;
for (int i=1; i<=n; i++)
{
char c;
cin>>c;
pref1[i] = pref1[i-1];
pref2[i] = pref2[i-1];
pref3[i] = pref3[i-1];
if (c == 'J')
{
pref1[i]++;
}
else if (c == 'O')
{
pref2[i]++;
}
else
{
pref3[i]++;
}
}
int mn = 1e9;
for (int i=1; i<=n; i++)
{
int st = pref1[i];
int poz2;
if (pref1[i-1] == pref1[i])
{
continue;
}
poz2 = bin_search1(st+k-1);
if (poz2 == -1)
{
continue;
}
st = pref2[poz2];
int poz3 = bin_search2(st+k);
if (poz3 == -1)
{
continue;
}
st = pref3[poz3];
int kraj = bin_search3(st+k);
if (kraj == -1)
{
continue;
}
mn = min(mn, kraj - i + 1 - 3 * k);
}
if (mn == 1e9)
{
cout<<-1;
return 0;
}
cout<<mn;
}
