#include <bits/stdc++.h>
using namespace std;

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for(int64 i = 0;i < (n);i++)
#define FOR(i, a, b) for(int64 i = (a);i < (b);i++)
#define all(x) x.begin(),x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template<typename A, typename B>inline void chmin(A &a, B b){if(a > b) a = b;}
template<typename A, typename B>inline void chmax(A &a, B b){if(a < b) a = b;}

template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
	return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value!=0>::type
fill_v(U &u,const V... v){u=U(v...);}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value==0>::type
fill_v(U &u,const V... v){
	for(auto &e:u) fill_v<T>(e,v...);
}

int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int64 N, M;
	cin >> N >> M;
	using PIC = pair<int64, char>;
	vector<PIC> cond(M);
	REP(i, M) {
	  cin >> cond[i].fs >> cond[i].sc;
	  cond[i].fs--;
	}
	string res(N, '0'); res[0] = '1';
	vector<char> changed(N, '_');
	REP(i, M) {
	  if (changed[cond[i].fs] == '_' || changed[cond[i].fs] == cond[i].sc) {
	    res[cond[i].fs] = cond[i].sc;
	    changed[cond[i].fs] = cond[i].sc;
	  } else {
	    cout << -1 << endl;
	    return 0;
	  }
	}
	if (res[0] == '0') {
	  cout << -1 << endl;
	  return 0;
	}
	cout << res << endl;
}
