From Arotou to Ansongo and Gao, in Mali, will be a rought journey that take two days by road. The "best" option is a STNV, that every Monday and Thursday a big truck leave from Niamey for try to cross the border. Another option is, if you are lucky, find some truck or public Land Rover that is going to Mali (price is around 8000 CFA to Gao, depend is you travel in the cabin or at the back on top of the merchandise) The vehicle will be crowded and the journey will be hard. Enjoy it!!!