import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.util.HashMap;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.Collections;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        CGuessTheNumber solver = new CGuessTheNumber();
        solver.solve(1, in, out);
        out.close();
    }

    static class CGuessTheNumber {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            HashMap<Integer, List<Integer>> map = new HashMap<>();
            for (int i = 0; i < m; i++) {
                int a = in.nextInt();
                int b = in.nextInt();
                if (!map.containsKey(a))
                    map.put(a, new ArrayList<>());
                int f = 0;
                for (int h : map.get(a))
                    if (h == b)
                        f++;
                if (f == 0)
                    map.get(a).add(b);
            }
            String s = "";
            int c = 0;
            if (n == 1 && !map.containsKey(1)) {
                out.println(0);
                return;
            }
            if (n == 1 && map.get(1).size() > 0 && map.get(1).get(0) == 0) {
                out.println(0);
                return;
            }
            for (int i = 1; i <= n; i++) {
                if (!map.containsKey(i)) {
                    if (i == 1)
                        s += "1";
                    else
                        s += "0";
                } else {
                    Collections.sort(map.get(i));
                    s += map.get(i).get(0);
                    if (map.get(i).size() > 1)
                        c++;
                    if (i == 1 && map.get(i).get(0) == 0)
                        c++;
                }
            }
            if (c == 0)
                out.println(s);
            else
                out.println(-1);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }
}



Measured memory_bytes: 23380