/*****from dust I have come, dust I will be*****/
#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define int long long
#define forn(i,n) for(int i=0;i<n;i++)
int dx[8] = { 1, 0, -1, 0, -1, 1, -1, 1 };
int dy[8] = { 0, 1, 0, -1, -1, 1, 1, -1 };
int ceil_div(int a, int b) {return a % b == 0 ? a / b : a / b + 1;}
const int MOD = 1000000007;
//const int MOD = 998244353;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds; // find_by_order, order_of_key
/*
int k = A.order_of_key(p[i].second);
A.erase(A.find_by_order(k));
erase element from pbds multiset
*/
void solve(){
int k,n; cin >> n>> k;
string s; cin >> s;
int ans = 1e18;
vector<int>j, o, i;
forn(x,n){
if (s[x] == 'J') j.push_back(x);
else if (s[x] == 'O') o.push_back(x);
else i.push_back(x);
}
int p1 = 0, p2 = k-1, x = 0;
while (p2 < (int)j.size()){
x = (p2 - p1 + 1)-k;
auto it = upper_bound(o.begin(), o.end(), j[p2]);
int lo = it - o.begin();
int ro = lo + (k - 1);
if (ro >= (int)o.size()){
p1++; p2++;
continue;;
}
x += (ro - lo + 1) - k;
it = upper_bound(i.begin(), i.end(), o[ro]);
int li = it - i.begin();
int ri = li + (k - 1);
if (ri >= (int)i.size()){
p1++; p2++;
continue;;
}
x += (ri - li + 1) - k;
//cout <<i[ri]-j[p1]+1 - k*3<<'\n';
ans = min(ans, (i[ri]-j[p1]+1 - k*3));
// cout <<j[p1]<<' '<<j[p2]<<'\n';
// cout <<o[lo]<<' '<<o[ro]<<'\n';
// cout <<i[li]<<' '<<i[ri]<<'\n';
p1++; p2++;
}
if (ans == 1e18) ans = -1;
cout <<ans<<'\n';
}
int32_t main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int t = 1; //cin >>t;
for (int i=1;i<=t;i++){
solve();
}
return 0;
}
