n, m = map(int, input().split())
ans = [0]*n

for _ in range(m):
    s, c = map(int, input().split())
    if ans[s-1] != 0 and ans[s-1] != c:
        print(-1)
        exit()
    ans[s-1] = c

if len(str(int(''.join(map(str, ans))))) != n:
    print(-1)
else:
    print(*ans, sep='')
