 #include<bits/stdc++.h>

using namespace std;

int main(){
  int N,M;
  cin >> N >> M;
  vector<int> s(M);
  vector<int> c(M);

  for(int i = 0; i < M; i++){ cin >> s[i] >> c[i]; if(s[i] == 1 && c[i] == 0 && N != 1){ cout << -1; return 0;}

  for(int i = 0; i < M; i++){
    for(int j = 0; j < M; j++){
      if(s[i] == s[j] && c[i] != c[j]){ cout << -1; return 0; }

    }
  }
  vector<int> z(N);
  for(int i = 0; i < N; i++){
    z[i] = 0;
  }
  for(int i = 0; i < M; i++){
    z[s[i]] = c[i];
  }
  double ans = 0;
  for(int i = 1; i < N + 1; i++){
    ans += (double)z[i]*pow(10.0,(double)(N - i));
  }
  int t = (int) ans;
  int count = 0;
  for(int i = 0; i < N; i++){
    if(t != 0 && count != 0){t /= 10;
    count++;
    }
  }
  if(count != N){cout << -1; return 0;}

  cout << ans << endl;

  return 0;
}



