#include<bits/stdc++.h>
#include<queue>
#define ll long long
using namespace std;
ll n, k, dem = 0, cnt[5], pfs[(int)2e5+5][5];
char a[5] = {'J', 'O', 'I'};
string s;
ll bs(ll x, ll y, ll l, ll r){
ll mid;
while(l <= r){
mid = (l+r)/2;
if(pfs[mid][y] >= x + k) r = mid-1;
else l = mid+1;
}
return l;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin>>n>>k>>s;
s = " "+s;
for(int i = 1; i <= n; i++){
for(int j = 0; j < 3; j++){
if(s[i] == a[j]){
pfs[i][j] = pfs[i-1][j] + 1;
}
else pfs[i][j] = pfs[i-1][j];
}
}
// for(int i = 1; i <= n; i++) cout<<pfs[i][0]<<" ";
//
// cout<<endl;
ll start = 1, i, j, ans = 0, x, ds = 1e18;
bool flag = true;
while(flag == true){
i = start;
// cout<<i<<endl;
ans = 0; x = 0;
for(int t = 0; t < 3; t++){
i = bs((-1)*k+1, t, i, n);
if(t == 0) x = i-1;
j = bs(pfs[i-1][t], t, i, n);
// cout<<i<<" "<<j<<" ";
ans += i - x - 1 + j - i + 1 - k;
// cout<<ans<<endl;
if(i > n || j > n){
flag = false;
ans = 1e18;
// cout<<"!! "<<i<<" "<<j;
break;
}
i = j+1;
x = j;
}
// cout<<ans<<endl<<endl;
ds = min(ds, ans);
if(flag == false) break;
// if(start == 3) break;
start++;
}
if(ds != 1e18) cout<<ds;
else cout<<-1;
}
