题解 | #小球投盒#

import java.io.*;
import java.util.*;
import java.math.BigInteger;

public class Main {
    static void solve() {
        int n = in.nextInt(), m = in.nextInt();
        HashMap<Integer,Integer> mp1 = new HashMap<>();
        HashMap<Integer,Integer> mp2 = new HashMap<>();
        for(int i=1;i<=m;i++) {
            int op = in.nextInt(), x = in.nextInt();
            if(op==1) {
                mp1.put(x,1);
                if(mp2.containsKey(x)||mp1.size()==n){
                    out.println(i);
                    return;
                }
            }else {
                mp2.put(x,1);
                if(mp1.containsKey(x)||mp2.size()>1) {
                    out.println(i);
                    return;
                }
            }
        }
        out.println(-1);
    }

    public static void main(String[] args) {
        solve();
        out.flush();
    }

    static FastReader in = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);

    static class FastReader {
        static BufferedReader br;
        static StringTokenizer st;

        FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            String str = "";
            while (st == null || !st.hasMoreElements()) {
                try {
                    str = br.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
                st = new StringTokenizer(str);
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }
    }
}