This is a 4 bit adder (obviously). This is a really low-grade calculator that cannot add numbers larger than 16 (in binary). If you want to check the validity of the calculator, here is a list of values for binary.
1 = 0001 , 4= 0100 , 7=0111 , 10 = 1010
2= 0010 , 5= 0101 , 8=1000 , 11= 1011
3= 0011 , 6= 0110 , 9= 1001 , 12= 1100