a = 0;
b = 0;
----------
a = -1;
b = 1;
----------
a = 1;
b = 1;
----------
a = -2;
b = 2;
----------
a = 2;
b = 2;
----------
a = -3;
b = 3;
----------
a = 3;
b = 3;
----------
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