First I got WA on test 10.Because my DP solution is wrong:
this is AC solution:
        tt[0].a=tt[0].b=0;
    for(int i=1;i<=k;i++)
    {
        scanf("%d%d",&tt[i].a,&tt[i].b);
    }
    sort(tt,tt+k+1,cmp);
    memset(dp,0,sizeof(dp));
    for(int i=1;i<=k;i++)
    {
        for(int j=0;j<i;j++)
        {
            if(tt[j].a<tt[i].a&&tt[j].b<tt[i].b)
            {
                dp[i]=max(dp[j]+1,dp[i]);
            }
        }
    }

this is WA solution
    for(int i=1;i<=k;i++)
    {
        scanf("%d%d",&tt[i].a,&tt[i].b);
    }
    sort(tt+1,tt+k+1,cmp);
    memset(dp,0,sizeof(dp));
        dp[1]=1;
    for(int i=2;i<=k;i++)
    {
        for(int j=1;j<i;j++)
        {
            if(tt[j].a<tt[i].a&&tt[j].b<tt[i].b)
            {
                dp[i]=max(dp[j]+1,dp[i]);
            }
        }
    }
sorry for my poor English .hope to help~