CF1846A Rudolph and Cut the Rope

There are $ n $ nails driven into the wall, the $ i $ -th nail is driven $ a_i $ meters above the ground, one end of the $ b_i $ meters long rope is tied to it. All nails hang at different heights one above the other. One candy is tied to all ropes at once. Candy is tied to end of a rope that is not tied to a nail. To take the candy, you need to lower it to the ground. To do this, Rudolph can cut some ropes, one at a time. Help Rudolph find the minimum number of ropes that must be cut to get the candy. The figure shows an example of the first test: 

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