#include<bits/stdc++.h>
using namespace std;
ostream& operator<<(ostream &out, string str) {
for(char c : str) out << c;
return out;
}
template<class L, class R> ostream& operator<<(ostream &out, pair<L, R> p) {
return out << "(" << p.first << ", " << p.second << ")";
}
template<class T> auto operator<<(ostream &out, T a) -> decltype(a.begin(), out) {
out << "{";
for(auto it = a.begin(); it != a.end(); it = next(it))
out << (it != a.begin() ? ", " : "") << *it;
return out << "}";
}
void dump() { cerr << "\n"; }
template<class T, class... Ts> void dump(T a, Ts... x) {
cerr << a << ", ";
dump(x...);
}
#ifdef DEBUG
# define debug(...) cerr << "[" #__VA_ARGS__ "]: ", dump(__VA_ARGS__)
#else
# define debug(...) false
#endif
#define REP(i, n) for(int i = 0; i < n; i++)
#define FOR(i, a, b) for(int i = a; i <= b; i++)
#define ST first
#define ND second
template<class T> int size(T && a) { return a.size(); }
using LL = long long;
using PII = pair<int, int>;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, k;
string str;
cin >> n >> k >> str;
vector<vector<int>> pos(3);
vector<int> last(4, -1e9);
map<char, int> id = {{'J', 0}, {'O', 1}, {'I', 2}};
int ans = 1e9;
REP(i, n) {
last[0] = i;
int x = id[str[i]];
pos[x].emplace_back(last[x]);
if(size(pos[x]) >= k)
last[x + 1] = pos[x][size(pos[x]) - k];
ans = min(ans, i - last[3] + 1 - 3 * k);
}
if(ans > n) ans = -1;
cout << ans << "\n";
}
