#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <cstring>
#include <iterator>
#include <random>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
const ll dim = 5*1e6+7;
//const ll mod = 1e9 + 7;
const ll inf = 1e18 + 77;
#define endl "\n"
#define fi first
#define pb push_back
#define se second
#define vll vector<ll>
ll n, m;
pll a[dim];
ll op1[dim], pr[4][dim], pos[dim];
ll b[dim];
int main() {
ll k, t, u0, v0;
string s1;
cin>>n>>k;
cin>>s1;
ll cnt1=0;
for(int i=1; i<=n; i++){
for(int j=1; j<=3; j++)pr[j][i]=pr[j][i-1];
if(s1[i-1]=='J')pr[1][i]++;
else if(s1[i-1]=='I')pr[3][i]++;
else{
pr[2][i]++;
cnt1++;
pos[cnt1]=i;
// cout<<pos[cnt1]<<" ";
}
}
//cout<<endl;
ll ans=inf;
for(int i=1; i<=n; i++){
ll a1=0;
ll a2=0;
ll a3=0;
if(s1[i-1]=='O'){
ll cur=pr[2][i];
if(cur+k-1>cnt1)continue;
ll p2=pos[cur+k-1];
ll p1=i;
a2=(p2-p1+1-k);
ll l=p2+1;
ll r=n;
while(r-l>=1){
ll mid=(l+r)/2;
if(pr[3][mid]-pr[3][p2]>=k){
r=mid;
}
else{
l=mid+1;
}
}
if(pr[3][l]-pr[3][p2]<k)continue;
a3=(l-p2-k);
l=0;
r=p1-1;
while(r-l>=1){
ll mid=(l+r+1)/2;
if(pr[1][p1]-pr[1][mid]<k){
r=mid-1;
}
else{
l=mid;
}
}
if(pr[1][p1]-pr[1][l]<k)continue;
a1=p1-l-1-k;
ans=min(ans, a1+a2+a3);
}
}
if(ans==inf)cout<<-1<<endl;
else cout<<ans<<endl;
return 0;
}
