Chef Birds

In chef-world (The world where our chef is God and it consists of chef-sky, chef-earth, chef-birds and obviously the chef himself), there are $N$ chef-birds. All of them will start flying from $0^{th}$ second with the same speed in chef-sky, the $i^{th}$ bird has to visit the chef-earth at every ${a_i}^{th}$ second (at $a_i$, $2* a_i$, $3*a_i$, ....) during her life. On chef-earth, at any insta