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

int main() {
    int n, m;
    cin >> n >> m;
    int res = -1;
    string current;
    for(int i = 0; i < 1000; i++){
        current = to_string(i);
        if(current.size != n) continue;
        int j = 0;
        for(int j = 0; j < m; j++){
            int s;
            cin >> s, s--;
            char c;
            cin >> c;
            if(current[s] != c) break;
        }
        if(j + 1 == m){
            res = i;
            break;
        }
    }
    cout << res << endl;
    return 0;
}