def main():
 
    N, M = map(int, input().split())
    sc = []
    for _ in range(M):
        s, c = map(int, input().split())
        sc.append([s, c])
    if N == 1:
        return sc[0][1]
 
    for v in range(pow(10, N-1), pow(10, N)):
        t = str(v)
        if all(t[s-1] == str(c) for s, c in sc):
            return v
    return -1
 
if __name__ == '__main__':
    print(main())