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We measure the tt production cross section in pp collisions at √s = 1.96 TeV in the lepton + jets channel. Two complementary methods discriminate between signal and background: b tagging and a kinematic likelihood discriminant. Based on 0.9 fb-1 of data collected by the D0 detector at the Fermilab Tevatron Collider, we measure σtt = 7.62 ± 0.85 pb, assuming the current world average mt = 172.6 GeV. We compare our cross section measurement with theory predictions to determine a value for the top-quark mass of 170 ± 7 GeV.