#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <bitset>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef long long LL;

typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<string, int> PSI;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a); i<(b); i++)
#define REP(i,n)  for(int i=0; i<(n); i++)

//constant
//--------------------------------------------
const double PI  = acos(-1.0);

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
const LL MOD = 1e9+7; 
//vector <vector<int>> x(20, vector<int> (20)), y(20,vector<int> (20));
/*int counter_1 (int x) {
    if(x == 0) return 0;
    return counter_1(x >> 1) + (x & 1);
}*/

//int Counter(int x)
/*int factorial(int n) {
    if (n > 0) {
        return n * factorial(n - 1);
    } else {
        return 1;
    }
}*/
/*bool pairCompare(const P& firstElof, const P& secondElof)
{
    return firstElof.second > secondElof.second;
}*/


/*long nCr(int n, int r)
{
       if(n==r) return dp[n][r] = 1;
       if(r==0) return dp[n][r] = 1;
       if(r==1) return dp[n][r] = n;
       if(dp[n][r]) return dp[n][r];
       return dp[n][r] = nCr(n-1,r) + nCr(n-1,r-1);
}*/
int main(){
    int n, m;
    cin >> n >> m;
    vector<PII> c(m);
    vector<bool> cnt(n, false);
    REP(i, m){
        int x, y;
        cin >> x >> y;
        c[i] = MP(x, y); 
        //cout << c[i].first << ' ' << c[i].second << endl;
        cnt[x-1]=true;
    }
    bool flg=true;
    sort(c.begin(), c.end());
    REP(i, m){
        if(c[i].first==1&&c[i].second==0&&n!=1){
            flg=false;
            break;
        }
        REP(j, m){
            if(c[i].first==c[j].first&&c[i].second!=c[j].second){
                flg=false;
                break;
            }
        }
    }
    if(flg==false){
        cout << -1 << endl;
    }else{
        REP(i, n){
            if(cnt[i]==true){
                REP(j, m){
                    if(c[j].first==i+1){
                        cout << c[j].second;
                        break;
                    }
                }
            }else{
                if(i==0&&n!=1) cout << 1;
                else cout << 0;
            }
        }
        cout << endl;
    }    
}


Measured memory_bytes: 256