n, m = map(int, raw_input().strip().split())

f = dict()

for i in xrange(m):
    s, c = map(int, raw_input().strip().split())
    try: f[s - 1].append(c)
    except: f[s - 1] = [c]

for key in f:
    f[key] = list(set(f[key]))
    if len(f[key]) > 1:
        print -1
        exit()

for i in xrange(1000):
    X = str(i)

    if len(X) != n:
        continue

    flag = 0
    for key in f:
        if key >= len(X):
            flag = 1
            break
        if X[key] != str(f[key][0]):
            flag = 1
            break

    if flag == 0 and len(X) == n:
        print X
        exit()

print -1
    
