#include <bits/stdc++.h>
using namespace std;
#define all(x) (x).begin(), (x).end()
#define ll long long int
#define pii pair <int, int>
#define pb push_back
#define ff first
#define ss second
const int N = 2e5+5;
int n, k, p[N][3];
string s;
int F(int i, int t){
if (i == -1) return -1;
int l = i, r = n, pos = -1;
while(l <= r){
int md = (l+r)>>1;
if (p[md][t]-p[i][t] < k) l = md + 1;
else{
pos = md;
r = md - 1;
}
}
return pos;
}
int main ()
{
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> k >> s;
s = '#' + s;
for (int i = 1; i <= n; i++){
for (int j = 0; j < 3; j++) p[i][j] = p[i-1][j];
if (s[i] == 'J') p[i][0]++;
if (s[i] == 'O') p[i][1]++;
if (s[i] == 'I') p[i][2]++;
}
const int inf = 1e9;
int mn = inf;
for (int i = 1; i <= n; i++){
if (s[i] != 'J') continue;
int r = F(F(F(i-1, 0), 1), 2);
if (r == -1) continue;
mn = min(mn, (r-i+1)-3*k);
}
if (mn == inf) mn = -1;
cout << mn;
}
