///TRAN THAI BAO :3
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <deque>
using namespace std;
#define maxN 200007
int n, k;
string s;
int a[maxN] = {0};
int goTo[maxN][3] = {0};
void xuLy(int ty)
{
deque<int>q;
for(int i = 0; i <= n; i++)
{
if(a[i] == ty)
q.push_back(i);
while(q.size() > k)
q.pop_front();
if(q.size() < k)
goTo[i][ty] = -1;
else goTo[i][ty] = q.front();
}
}
void solve()
{
cin >> n >> k >> s;
s = ' ' + s;
a[0] = -1;
for(int i = 1; i <= n; i++)
if(s[i] == 'J')
a[i] = 0;
else if(s[i] == 'O')
a[i] = 1;
else a[i] = 2;
for(int i = 0; i < 3; i++)
xuLy(i);
int ans = 2*n;
for(int i = 0; i <= n; i++)
{
int cur = i;
for(int j = 2; j >= 0; j--)
{
cur = goTo[cur][j]-1;
if(cur == -1)
break;
}
if(cur == -1)
continue;
ans = min(ans, i - cur - 3*k);
}
if(ans == 2*n)
cout << -1;
else cout << ans;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
solve();
return 0;
}
