A man, aged 54, complains of constant dull ache in mesogastric area, weight loss, dark blood admixtures in feces, constipations. In the course of the year the weight loss is up to 10 kg. In blood: RBC - 3,5*1012/L, Hb - 87 g/L, WBC – 12,6*109/L, stab shift, ESR – 43 mm/h. What is the most probable preliminary diagnosis?