#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  scanf("%d %d", &n, &m);
  vector<int> p{1};
  while (p.size() < n) p.insert(p.begin(), p[0] * 10);
  map<int, int> s;
  int ans = 0;
  while (m--) {
    int a, b;
    scanf("%d %d", &a, &b);
    if (s.count(a)) {
      if (s[a] != b) return printf("-1\n"), 0;
    } else {
      s[a] = b;
      ans += b * p[a - 1];
      if (a == 1 and b == 0 and n > 1) return printf("-1\n"), 0;
    }
  }
  if (prev(s.end())->second == 0) {
    ans += p[prev(s.end())->first - 2];
  }
  printf("%d\n", ans);
}
