# input
N, M = map(int, input().split())
s = [0]*M
c = [0]*M

if(M==0 and N==1):
    print("0")
elif(M==0 and N>1):
    print("-1")
else:
    for i in range(M):
        s[i], c[i] = map(int, input().split())

    ans = [-1]*N
    b = True

    for i in range(M):
        if(ans[s[i]-1] != -1 and ans[s[i]-1] != c[i]):
            b = False
            break
        ans[s[i]-1] = c[i]

    if(b==False or ans[0]==0):
        b = False
    else:
         for i in range(len(ans)):
                if(ans[i] == -1):
                    ans[i] = 0
    if(b):
        ans_print = ""
        for i in range(len(ans)):
            ans_print += str(ans[i])
        print(ans_print)
    else:
        print("-1")