#include <bits/stdc++.h>
using namespace std;
#define print(l) for(auto i:l) cout<<i<<" ";cout<<endl;
#define input(t,l,n) vector<t>l(n);for(int i = 0;i<n;i++)cin>>l[i];
#define int long long
#define pb push_back
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define all(l) l.begin(),l.end()
#define pii pair<int,int>
#define fi first
#define se second
const int M = 1e9+7;
const int inf = 1e18;
int bp(int x, int y, int p){
int res = 1;
x = x % p;
while (y > 0) {
if (y & 1)
res = (res * x) % p;
y = y >> 1;
x = (x * x) % p;
}
return res;
}
int MI(int n, int p){
return bp(n, p - 2, p);
}
int mul(int x,int y, int p){
return x * 1ull * y % p;
}
int di(int x,int y, int p){
return mul(x, MI(y, p), p);
}
int n , m , k , q,d;
void solve(int testcase_number){
cin>>n>>m;
string s;
cin>>s;
vector<int> ind[3];
for(int i = 0;i<n;i++){
if(s[i] == 'J'){
ind[0].pb(i);
}
else if(s[i] == 'O'){
ind[1].pb(i);
}
else{
ind[2].pb(i);
}
}
ind[0].pb(inf);
ind[1].pb(inf);
ind[2].pb(inf);
// print(ind[2]);
int ans = inf;
for(int i =0 ;i+m-1<ind[0].size();i++){
int fj = ind[0][i];
int lj = ind[0][i+m-1];
int t1 = lower_bound(all(ind[1]),lj)-ind[1].begin();
int fo = ind[1][t1];
int lo = ind[1][min((int)ind[1].size()-1,t1+m-1)];
int t = lower_bound(all(ind[2]),lo)-ind[2].begin();
int fi = ind[2][t];
int li = ind[2][min((int)ind[2].size()-1,t+m-1)];
ans = min(ans,li-fj+1 - 3*m);
ans = max(ans,-1ll);
// cout<<t1<<" "<<t<<endl;
// cout<<ans<<" "<<fj<<" "<<lj<<" "<<fo<<" "<<lo<<" "<<fi<<" "<<li<<endl;
}
cout<<ans<<endl;
}
signed main(){
ios::sync_with_stdio(0);//DO NOT USE IN INTERACTIVE
cin.tie(0), cout.tie(0);
cout << fixed<<setprecision(9);
int t = 1;
// cin>>t;
for(int i = 1;i<=t;i++){
solve(i);
}
}
