f(1) = 160
f(2) is -2 times the previous value
f(2) = -2 *f(1) = -2 * 160 = -320
f(3) = -2 * f(2) = -2 *(-320) = 640
f(4) = -2 * f(3) = -2 * 640 = -1280
f(4) = -1280
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