/*#pragma GCC optimize("O3")
#pragma GCC optimization("Ofast,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")*/
#include <bits/stdc++.h>
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>*/
#define ll long long
#define ll1 long long
#define ull unsigned long long
#define dou long double
#define str string
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll, ll>
#define vpll vector<pll>
#define vbool vector<bool>
#define vstr vector<str>
#define vvll vector<vll>
#define pb push_back
#define pf push_front
#define endl "\n"
#define fr first
#define se second
// #define sortcmp(a) sort(a.begin(), a.end(), cmp)
#define sort(a) sort(a.begin(), a.end())
#define reverse(a) reverse(a.begin(), a.end())
#define speed ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define INF 1000000000000000007
#define ordered_set tree<ll, null_type, less_equal<ll>, rb_tree_tag, tree_order_statistics_node_update>
using namespace std;
//using namespace __gnu_pbds;
mt19937 rng(time(0));
ll randll(ll l, ll r) {
return uniform_int_distribution<ll>(l, r)(rng);
}
int main(){
speed;
srand(time(0));
ll n, k;
cin >> n >> k;
str s;
cin >> s;
vvll p(3, vll(n+7, 0));
for (int i = 0; i < n; i ++) {
p[0][i+1] = p[0][i];
if (s[i] == 'J') p[0][i+1] ++;
}
for (int i = 0; i < n; i ++) {
p[1][i+1] = p[1][i];
if (s[i] == 'O') p[1][i+1] ++;
}
for (int i = 0; i < n; i ++) {
p[2][i+1] = p[2][i];
if (s[i] == 'I') p[2][i+1] ++;
}
ll minl = INF;
for (int i = 0; i < n; i ++) {
if (s[i] != 'J') continue;
ll x = i;
bool q = 0;
for (int j = 0; j < 3; j ++) {
ll l = x, r = n;
while (r-l > 1) {
ll md = (l+r)/2;
if (p[j][md+1]-p[j][x] < k) l = md;
else r = md;
}
if (r == n) {
q = 1;
break;
}
x = r;
}
if (q) break;
minl = min(minl, x-i+1-k*3);
}
if (minl == INF) cout << -1 << endl;
else cout << minl << endl;
}
