1+3 = 4
1-3 = -2
1*3 = 3
1/3 = 0
1+1 = 2
1* (1+3) = 4
1*  1+3  = 4
5+2 = 7
5-2 = 3
5*2 = 10
5/2 = 2
5+1 = 6
5* (5+2) = 35
5*  5+2  = 27