...7 +...a = ...13 => a=6; 7+6=13
...b0+ ...50 + 13 = ....13 =>
...b0+ ...50 = ....00 => b=5; 5+5=10
..c00+..c00 = ..400+113 => c+c=4; c=2
b257+d255=6513
257+255=513
b256+d255-513 = 6000
b000+d000=6000
5000+d000=6000
d=1
a=6 b=5 c=2 d=1
a+c-d+b = 6+2-1+5 = 12
