how many calories should I eat?

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Hi everybody!I have lost 12 kg last year and I have recently gained back 3 kg after a back injury! I ' m desperate and I need to something about it.I am at 71 kg and I 'm 1,63 m tall.I ' m 35 years old and have a sedentary job.I can 't exercise for 2 or 3 months and I ' m afraid I 'll put all the weight back on.How many calories should I eat in order to lose the 3 kilos again?