date = input().split()
N = date[0]
M = date[1]
dict = {}
sum = 0
flag = 0
for i in  range(int(M)):

    date = input().split()
    s = date[0]
    c = date[1]
    if s in dict.keys():
        if dict[s] != c:
            print("-1")
            flag = 1
            break
    else:
        dict[s] = c
dict = sorted(dict.items(),key=lambda x:x[0])
for i in dict:
    if int(i[1]) == 0 and int(N)>1:
        print(-1)
        flag = 1
        break
    else:
        sum += 10**(int(N)-int(i[0]))*int(i[1])

if flag == 0:
    print(sum)
