I tried all test cases which were written in the discussion part and my program gives the correct answer for all of them. How to find my error:

#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define endl "\n" /*DELETE IT ON INTERACTIVES!!*/
#define mod 1000000007
#define int long long
#define double long double
#define all(x) x.begin(), x.end()

const int N = 1e3+5;
vector<int> adj[N],marked(N,0),killed(N,0),vis_1(N,0),torun(N,1),vis_2(N,0);
int ok;

void dfs(int node){

    vis_1[node] = 1;
    if(killed[node]) ok = 1;

    for(auto go : adj[node]){
        if(!vis_1[go]){
            dfs(go);
        }
    }
}

void mark(int node){

    marked[node] = 1;
    vis_2[node] = 1;

    for(auto go : adj[node]){
        if(!vis_2[go]){
            mark(go);
        }
    }
}

void solve(){

    int n; cin>>n;

    string s,t;

    while(cin>>s){
        if(s == "BLOOD") break;
        cin>>t;
        adj[(int)s[0] - '0'].pb((int)t[0] - '0');
        torun[(int)t[0] - '0'] = 0;
    }

    int a;

    while(cin>>a){
        killed[a] = 1;
    }

    for(int i=1;i<=n;i++){
        if(torun[i]){
            dfs(i);
            if(ok){
                mark(i);
            }
            fill(all(vis_1),0);
            fill(all(vis_2),0);
            ok = 0;
        }
    }

    int hehe = 0;

    for(int i=1;i<=n;i++){
        if(!marked[i]){
            hehe = 1;
            cout<<i<<' ';
        }
    }

    if(!hehe) cout<<0;
}

int32_t main(){

    cin.tie(0); ios::sync_with_stdio(0);

    int t=1; //cin>>t;

    while(t--) solve();

    return 0;
}