#include<bits/stdc++.h> //:3
using namespace std;
typedef long long ll;
#define all(a) (a).begin(), (a).end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define pi pair<int, int>
#define sz(x) (int)((x).size())
#define int long long
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
const ll inf = 2e9;
const ll mod = 1e9 + 7;
const int N = 2e5 + 11;
const int X = 1e6;
const ll INF64 = 3e18 + 1;
const double eps = 1e-14;
const double PI = acos(-1);
//ifstream in(".in");
//ofstream out(".out");
int n, k, J[N], O[N], I[N];
void solve(){
cin >> n >> k;
string s;
cin >> s;;
s = '/' + s;
for(int i = 1; i <= n; i++){
J[i] = J[i - 1] + (s[i] == 'J');
O[i] = O[i - 1] + (s[i] == 'O');
I[i] = I[i - 1] + (s[i] == 'I');
}
int j = 1, o = 1, i = 1, ans = n + 1;
for(int l = 1; l <= n; l++){
while(j <= n && (J[j] - J[l - 1]) < k){
j++;
}
if(j == n + 1)break;
while(o <= n && (O[o] - O[j]) < k){
o++;
}
if(o == n + 1)break;
while(i <= n && (I[i] - I[o]) < k){
i++;
}
if(i == n + 1)break;
ans = min(ans, i - l + 1 - 3*k);
//cout << i << ' ' << j << ' ' << i - l << '\n';
}
if(ans == n + 1)cout << "-1" << '\n';else cout << ans << '\n';
}
int32_t main(){
ios_base :: sync_with_stdio(0); cin.tie(0); cout.tie(0);
//cout << setprecision(6) << fixed;
int T = 1;
//cin >> T;
while(T--){
solve();
}
}
