N,M = map(int,input().split())
Output = [0] * N
num = ""

for i in range(M):
    s,c = map(int,input().split())
    if(Output[s-1]==0 or Output[s-1]>c):
        Output[s-1] = c
    if(c==0):
        print(-1)

if(Output[0]==0):
    print(-1)
else:
    for i in Output:
        if(i>9):
            print(-1)
        num += str(i)
    print(num)

