import sys
N, M = map(int, input().split())

#とりまぶっこむ 
SClis = [] 
for _ in range(M):
    a, b = map(int, input().split())
    SClis.append([a,b])

ans = [-1]*N

for i in range(M):
    singlelis = SClis[i]
    S =  (singlelis[0] - 1)
    C =  (singlelis[1])
    
    if ans[S] != -1 and ans[S] != C:
        print(-1)
        sys.exit()
    else:
        ans[S] = C


    
if N == 1:
    if ans[0] == -1:
        ans[0] = 0
        print(''.join(map(str, ans)))
        sys.exit()

if N != 1:
    if ans[0] == 0:
        print(-1)
        sys.exit()