Farmer John has N cows that need to be milked (1 <= N <= 10,000), each of which takes only one unit of time to milk. Being impatient animals, some cows will refuse to be milked if Farmer John waits too long to milk them. More specifically, cow i produces g\_i gallons of milk (1 <= g\_i <= 1000), but only if she is milked before a deadline at time d\_i (1 <= d\_i <= 10,000). Time starts at t=0, so at most x total cows can be milked prior to a deadline at time t=x. Please help Farmer John determine the maximum amount of milk that he can obtain if he milks the cows optimally. FJ有N(1 <= N <= 10,000)头牛要挤牛奶，每头牛需要花费1单位时间。 奶牛很厌烦等待，奶牛i在它的截止时间d\_i (1 <= d\_i <= 10,000)前挤g(1 <= g\_i <= 1000)的奶，否则将不能挤奶。时间t开始时为0，即在时间t=x时，最多可以挤x头奶牛。 请计算FJ的最大挤奶量。
\* Line 1: The value of N. \* Lines 2..1+N: Line i+1 contains the integers g\_i and d\_i.
\* Line 1: The maximum number of gallons of milk Farmer John can obtain.
4 10 3 7 5 8 1 2 1
There are 4 cows. The first produces 10 gallons of milk if milked by time 3, and so on. Farmer John milks cow 3 first, giving up on cow 4 since she cannot be milked by her deadline due to the conflict with cow 3. Farmer John then milks cows 1 and 2.