n, m = map(int, input().split())
ans = [-1]*n

for i in range(m):
    s, c = map(int, input().split())
    if ans[s-1] != -1 and ans[s-1] != c:
        print(-1)
        exit()
    ans[s-1] = c

for i in range(n):
    if ans[i] == -1:
        ans[i] = 0

if ans[0] == 0:
    print(-1)
    exit()

print(*ans, sep="")
