n, m = map(int, input().split())
s = []
c = []
for i in range(m):
    t, d = map(int, input().split())
    s.append(t)
    c.append(d)
a = -2
if m == 0:
    if n == 1:
        a = 0
    elif n == 2:
        a == 10
    else:
        a == 100
else:
    for i in range(m-1):
        for j in range(i+1, m):
            if s[i] == s[j]:
                if c[i] != c[j]:
                    a = -1
    if a == -2:
        if n == 1:
            a = c[0]
        if n == 2:
            for i in range(10, 100):
                b = 0
                for j in range(m):
                    if s[j] == 1:
                        if i//10 == c[j]:
                            b += 1
                    else:
                        if i%10 == c[j]:
                            b += 1
                if b == m:
                    a = i
                    break;
        if n == 3:
            for i in range(100, 1000):
                b = 0
                for j in range(m):
                    if s[j] == 1:
                        if i//100 == c[j]:
                            b += 1
                    elif s[j] == 1:
                        if (i//10)//10 == c[j]:
                            b += 1
                    else:
                        if i%10 == c[j]:
                            b += 1
                if b == m:
                    a = i
                    break;
    if a == -2:
        a = -1
print(a)