#include <bits/stdc++.h>
using namespace std;
long long low(vector<long long> a, long long b) {
  decltype(a)::iterator c = lower_bound(a.begin(), a.end(), b);
  long long d = c - a.begin();
  return d;
}
long long upp(vector<long long> a, long long b) {
  decltype(a)::iterator c = upper_bound(a.begin(), a.end(), b);
  long long d = c - a.begin();
  return d;
}
template <class T>
void cou(vector<vector<T>> a) {
  long long b = a.size();
  long long c = a[0].size();
  for (long long i = 0; i < b; i++) {
    for (long long j = 0; j < c; j++) {
      cout << a[i][j];
      if (j == c - 1)
        cout << endl;
      else
        cout << ' ';
    }
  }
}
long long wari(long long a, long long b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
long long keta(long long a) {
  long double b = a;
  b = log10(b);
  long long c = b;
  return c + 1;
}
long long souwa(long long a) { return a * (a + 1) / 2; }
long long gcm(long long a, long long b) {
  if (a % b == 0) return b;
  return gcm(b, a % b);
}
bool prime(long long a) {
  if (a < 2)
    return false;
  else if (a == 2)
    return true;
  else if (a % 2 == 0)
    return false;
  long double b = sqrt(a);
  for (long long i = 3; i <= b; i += 2) {
    if (a % i == 0) {
      return false;
    }
  }
  return true;
}
struct Union {
  vector<long long> par;
  Union(long long a) { par = vector<long long>(a, -1); }
  long long find(long long a) {
    if (par[a] < 0)
      return a;
    else
      return par[a] = find(par[a]);
  }
  bool same(long long a, long long b) { return find(a) == find(b); }
  long long Size(long long a) { return -par[find(a)]; }
  void unite(long long a, long long b) {
    a = find(a);
    b = find(b);
    if (a == b) return;
    if (Size(b) > Size(a)) swap<long long>(a, b);
    par[a] += par[b];
    par[b] = a;
  }
};
long long ketas(long long a) {
  string b = to_string(a);
  long long c = 0;
  for (long long i = 0; i < keta(a); i++) {
    c += b[i] - '0';
  }
  return c;
}
long long lcm(long long a, long long b) { return a / gcm(a, b) * b; }
bool fe(long long a, long long b) {
  a %= 10;
  b %= 10;
  if (a == 0) a = 10;
  if (b == 0) b = 10;
  if (a > b)
    return true;
  else
    return false;
}
long long INF = 1000000007;
struct edge {
  long long s, t, d;
};
vector<long long> mojisyu(string a) {
  vector<long long> b(26, 0);
  for (long long i = 0; i < a.size(); i++) {
    b[a[i] - 'a']++;
  }
  return b;
}
long long wa2(long long a) {
  if (a % 2 == 1) return a / 2;
  return a / 2 - 1;
}
bool compare_by_b(pair<long long, long long> a, pair<long long, long long> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}
signed main() {
  string x;
  long long N, M, X = 0, a;
  cin >> N >> M;
  vector<pair<long long, char>> s(M);
  for (long long i = 0; i < M; i++) cin >> s[i].first >> s[i].second;
  for (long long i = 0; i < N; i++) x.push_back('0');
  for (long long i = 0; i < M; i++) {
    if (x[s[i].first - 1] == s[i].second || x[s[i].first - 1] == '0') {
      x[s[i].first - 1] = s[i].second;
    } else {
      X = 1;
      break;
    }
  }
  if (!X && x[0] != '0') {
    a = atoi(x.c_str());
    cout << a << endl;
  } else
    cout << -1 << endl;
}
