#include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
using namespace std;
int n, k;

int gra[1010][1010];
bool vis[1010];
int ans = 0;

void dfs(int node, int oil, int cnt) {
    if(oil == 0) {
        if(cnt > ans) ans = cnt;
        return;
    }
    vis[node]=true;
    for(int i=0; i<n; i++) {
        if(node != i && gra[node][i]==1 && !vis[i]) {
            dfs(i, oil-1, cnt+1);
            oil--;
            vis[i]=false;
        }
    }
}

int main()
{
    scanf("%d %d", &n, &k);
    memset(gra, 0, sizeof(gra));
    for(int i=0; i<n-1; i++) {
        int cur;
        scanf("%d", &cur);
        gra[i+1][cur] = gra[cur][i+1] = 1;
    }
    memset(vis, false, sizeof(vis));
    dfs(0, 3, 0);
    cout << ans << endl;
    return 0;
}