from sys import stdin
import math

n, m = [int(x) for x in stdin.readline().rstrip().split()]

s = [0] * m
c = [0] * m
for i in range(m):
    s[i], c[i] = [int(x) for x in stdin.readline().rstrip().split()]

result = -1
for nn in range(10 ** n):
    ns = str(nn)
    ns = ns.zfill(n)
    flag = True
    for i in range(m):
        if ns[s[i]-1] == str(c[i]):
            continue
        else:
            flag = False
            break
    if flag:
        result = nn
        if n > 1 and result == 0:
            result = -1
        break

print(result)
        
