Having two buckets the tanks of 7 l and 4 l how it is possible to gain 5 l of water from the river? HELP PZh

On to take a bucket of 7 l to pour in a bucket 4 l then to pour out a half of it and will turn out in a bucket of 7l-3 l, and in a bucket of 4 l - 2 l (but it not precisely) Can be filled the first up to 1 liter and the second ponostyu
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