import sys
def input(): return sys.stdin.readline().strip()

def resolve():
    def main():

        n,m=map(int, input().split())
        l=[list(map(int,input().split())) for i in range(m)]
        if n==1 and m==0:
            print(0)
        else:


            for i in range(10**(n-1),10**n):
                flag = True
                for x,y in l:
                    z=str(i)
                    if int(z[x-1])!=y:
                        flag=False
                        break
                if flag:
                    return i

            return -1
    print(main())
resolve()