#include <bits/stdc++.h>
using namespace std;
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, -1, 0, 1};
int ddx[] = {-1, 0, 1, 0, -1, 1, -1, 1};
int ddy[] = {0, -1, 0, 1, 1, 1, -1, -1};
long long gcd(long long x, long long y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}
long long lcm(long long x, long long y) { return (x * y) / gcd(x, y); }
int main() {
  int n, m;
  cin >> n >> m;
  int s[5], c[5];
  for (int i = 0; i < m; ++i) cin >> s[i] >> c[i];
  int num[n];
  for (int i = 0; i < n; ++i) num[i] = -1;
  for (int i = 0; i < m; ++i) {
    if (num[s[i] - 1] == -1)
      num[s[i] - 1] = c[i];
    else if (num[s[i] - 1] == c[i])
      num[s[i] - 1] = c[i];
    else {
      cout << -1 << endl;
      return 0;
    }
  }
  if (n == 1 && num[0] == 0) {
    cout << 0 << endl;
  } else if (n != 1 && num[0] == 0) {
    cout << -1 << endl;
    return 0;
  }
  if (num[0] == -1) {
    cout << -1 << endl;
  } else {
    for (int i = 0; i < n; ++i) {
      if (num[i] == -1)
        cout << 0;
      else
        cout << num[i];
    }
    cout << endl;
  }
  return 0;
}
