#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
// #include <rail.h>
#include <cstdio>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x) cerr << #x <<" "; print(x); cerr << endl;
#else
#define dbg(x)
#endif
#define sz(x) (int((x).size()))
#define len(x) (int)x.length()
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define clr(x) (x).clear()
#define uniq(x) x.resize(unique(all(x)) - x.begin());
#define pb push_back
#define popf pop_front
#define popb pop_back
#define ld long double
#define ll long long
void print(long long t) {cerr << t;}
void print(int t) {cerr << t;}
void print(string t) {cerr << t;}
void print(char t) {cerr << t;}
void print(double t) {cerr << t;}
void print(unsigned long long t) {cerr << t;}
void print(long double t) {cerr << t;}
template <class T, class V> void print(pair <T, V> p);
template <class T> void print(vector <T> v);
template <class T> void print(set <T> v);
template <class T, class V> void print(map <T, V> v);
template <class T> void print(multiset <T> v);
template <class T> void print(T v[],T n) {cerr << "["; for(int i = 0; i < n; i++) {cerr << v[i] << " ";} cerr << "]";}
template <class T, class V> void print(pair <T, V> p) {cerr << "{"; print(p.first); cerr << ","; print(p.second); cerr << "}";}
template <class T> void print(vector <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(deque <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(set <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(multiset <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T, class V> void print(map <T, V> v) {cerr << "[ "; for (auto i : v) {print(i); cerr << " ";} cerr << "]";}
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define nl '\n'
// for random generations
mt19937 myrand(chrono::steady_clock::now().time_since_epoch().count());
// mt19937 myrand(131);
// for grid problems
int dx[8] = {-1,0,1,0,1,-1,1,-1};
int dy[8] = {0,1,0,-1,1,1,-1,-1};
long long ka() {
long long x = 0;
bool z = false;
while (1)
{
char y = getchar();
if (y == '-')
z = true;
else if ('0' <= y && y <= '9')
x = x * 10 + y - '0';
else
{
if (z)
x *= -1;
return x;
}
}
}
// lowest / (1 << 17) >= 1e5 / (1 << 18) >= 2e5 / (1 << 21) >= 1e6
void fastIO() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
}
// file in/out
void setIO(string str = "") {
fastIO();
if (str != "") {
freopen((str + ".in").c_str(), "r", stdin);
freopen((str + ".out").c_str(), "w", stdout);
} else if(str == "input") {
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
}
}
// Indexed Set
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
map<char, vector<int>> mp;
void solve_() {
int n, k; cin >> n >> k;
string sr; cin >> sr;
vector<int> p(n + 2, -1), s(n + 2, -1), pi(n + 2, 0);
for(int i = 0; i < n; i++) {
mp[sr[i]].push_back(i);
if(sr[i] == 'O') {
if(i == 0) {
pi[i] = 1;
} else {
pi[i] = pi[i - 1] + 1;
}
} else {
if(i == 0) {
pi[i] = 0;
} else {
pi[i] = pi[i - 1];
}
}
}
if(sz(mp['J']) < k) {
cout << -1 << '\n';
return;
}
if(sz(mp['O']) < k) {
cout << -1 << '\n';
return;
}
if(sz(mp['I']) < k) {
cout << -1 << '\n';
return;
}
for(int i = k - 1; i < sz(mp['J']); i++) {
p[mp['J'][i]] = mp['J'][i] - mp['J'][i - (k - 1)] - k + 1;
}
reverse(all(mp['I']));
for(int i = k - 1; i < sz(mp['I']); i++) {
s[mp['I'][i]] = abs(mp['I'][i] - mp['I'][i - (k - 1)]) - k + 1;
}
for(int i = n - 2; i >= 0; i--) {
if(s[i + 1] != -1) {
if(s[i] == -1) {
s[i] = s[i + 1] + 1;
} else {
if(s[i + 1] + 1 < s[i]) {
s[i] = s[i + 1] + 1;
}
}
}
}
int ans = INT32_MAX;
for(int i = k - 1; i < sz(mp['J']); i++) {
auto it = lower_bound(all(pi), pi[mp['J'][i]] + k);
if(it != pi.end()) {
int ind = it - pi.begin();
if(s[ind + 1] != -1) {
ans = min(ans, p[mp['J'][i]] + s[ind + 1] + ind - mp['J'][i] - k);
}
}
}
if(ans == INT32_MAX) {
ans = -1;
}
cout << ans << '\n';
}
int main() {
setIO();
auto solve = [&](int test_case)-> void {
while(test_case--) {
solve_();
}
};
int test_cases = 1;
// cin >> test_cases;
solve(test_cases);
return 0;
}
