#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline void smin(T &a, T b) {
  a = a < b ? a : b;
}
template <typename T>
inline void smax(T &a, T b) {
  a = a > b ? a : b;
}
void err(istream_iterator<string> it) { cout << endl; }
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << ", ";
  err(++it, args...);
}
template <typename T>
inline void Int(T &n) {
  n = 0;
  int f = 1;
  register int ch = getchar();
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') f = -1;
  for (; isdigit(ch); ch = getchar()) n = (n << 3) + (n << 1) + ch - '0';
  n = n * f;
}
template <typename T, typename TT>
inline void Int(T &n, TT &m) {
  Int(n);
  Int(m);
}
template <typename T, typename TT, typename TTT>
inline void Int(T &n, TT &m, TTT &l) {
  Int(n, m);
  Int(l);
}
const int mod = (int)1e9 + 7;
inline int add(int a, int b) {
  a += b;
  return a >= mod ? a - mod : a;
}
inline int sub(int a, int b) {
  a -= b;
  return a < 0 ? a + mod : a;
}
inline int mul(int a, int b) { return (long long)a * b % mod; }
const int inf = (int)2e9 + 5;
const long long Inf = (long long)2e18 + 5;
const int N = (int)2e5 + 5;
int solve() {
  int n, m;
  Int(n, m);
  std::set<int> g[n + 1];
  for (int i = 1; i <= m; i++) {
    int s, c;
    Int(s, c);
    g[s].insert(c);
  }
  for (int i = 1; i <= n; i++) {
    if ((int)g[i].size() > 1) return printf("-1\n");
  }
  if (n == 1) return printf("%d\n", *g[1].begin());
  if (*g[1].begin() == 0) return printf("-1\n");
  for (int i = 1; i <= n; i++) {
    if ((int)g[i].size() == 0)
      printf("0");
    else
      printf("%d", *g[i].begin());
  }
  printf("\n");
  return 0;
}
int main() {
  int test = 1, tc = 0;
  while (test--) {
    solve();
  }
  return 0;
}
