#include <bits//stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less<long long>, rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
tree_order_statistics_node_update>
ordered_multiset;
#define ll long long
#define ld long double
#define iloop(m, h) for (auto i = m; i != h; i += (m < h ? 1 : -1))
#define jloop(m, h) for (auto j = m; j != h; j += (m < h ? 1 : -1))
#define kloop(m, h) for (auto k = m; k != h; k += (m < h ? 1 : -1))
#define lloop(m, h) for (auto l = m; l != h; l += (m < h ? 1 : -1))
#define iloop_(m, h, g) for (auto i = m; i < h; i += g)
#define jloop_(m, h, g) for (auto j = m; j < h; j += g)
#define kloop_(m, h, g) for (auto k = m; k < h; k += g)
#define lloop_(m, h, g) for (auto l = m; l < h; l += g)
#define getchar_unlocked _getchar_nolock // comment before submission
#define pll pair<ll, ll>
#define plll pair<ll, pll>
#define pllll pair<pll, pll>
#define vll vector<ll>
#define qll queue<ll>
#define dll deque<ll>
#define pqll priority_queue<ll>
#define gll greater<ll>
#define INF 1000000000000000
#define MOD1 1000000007
#define MOD2 998244353
#define MOD3 1000000009
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ll n, m;
string s;
cin >> n >> m >> s;
ll j[n+1], o[n+1], i_[n+1];
j[0] = o[0] = i_[0] = 0;
iloop(1, n+1) {
j[i] = j[i-1];
o[i] = o[i-1];
i_[i] = i_[i-1];
if (s[i-1] == 'J') j[i]++;
else if (s[i-1] == 'O') o[i]++;
else i_[i]++;
}
ll cans = INF, nid;
iloop(1, n+1) {
//cout << i << " ";
if (i_[i] >= m) {
nid = lower_bound(i_, i_+n+1, i_[i]-m+1) - i_ - 1;
//cout << nid << " ";
if (o[nid] >= m) {
nid = lower_bound(o, o+n+1, o[nid]-m+1) - o - 1;
//cout << nid << " ";
if (j[nid] >= m) {
nid = lower_bound(j, j+n+1, j[nid]-m+1) - j - 1;
cans = min(cans, i-nid - 3*m);
//cout << nid << " ";
}
}
}
//cout << "\n";
}
cout << (cans == INF ? -1 : cans);
}
