#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const int INF = 1001001001;
vector<pair<int64_t, int64_t>> func(int64_t x) {
  vector<pair<int64_t, int64_t>> p;
  for (int64_t i = 2; i * i <= x; i++) {
    int cnt = 0;
    if (x % i == 0) {
      while (x % i == 0) {
        cnt++;
        x /= i;
      }
      p.push_back(make_pair(i, cnt));
    }
  }
  if (x != 1) {
    p.push_back(make_pair(x, 1));
  }
  return p;
}
int main() {
  int N, M;
  cin >> N >> M;
  int s[10];
  int c[10];
  vector<int> d(N);
  map<int, int> D;
  for (int i = 0; i < N; i++) {
    cin >> s[i] >> c[i];
  }
  for (int i = 0; i < N; i++) {
    if (s[i] == 1 && c[i] == 0) {
      cout << -1 << endl;
      return 0;
    } else if (D[s[i] - 1] >= 1 && d[s[i] - 1] != c[i]) {
      cout << -1 << endl;
      return 0;
    }
    D[s[i] - 1]++;
    d[s[i] - 1] = c[i];
  }
  for (int i = 0; i < N; i++) {
    cout << d[i];
  }
  cout << endl;
  return 0;
}
