
#include <iostream>

using namespace std;

int main(){
    int N,M;
    int S = 0;
    cin >> N >> M;
    int s[M];
    int c[M]
    for(int i = 0;i < N;i++){
        cin >> s[i] >> c[i];
        for(int j = 0;j < i;j++){
            if(s[i] == s[j] && c[i] == c[j]){
                cout << "-1" << endl;
            }
        }
        S = c[i] *  pow(10, (N + 1 - s[i]));
    }
    if(S / pow(10, N) == 0){
        cout << "-1" << endl;
    }else{
        cout << S << endl;
    }
}
