6667：Roundgod and Milk Tea

Roundgod is a famous milk tea lover at Nanjing University second to none. This year, he plans to conduct a milk tea festival. There will be $n$ classes participating in this festival, where the $i$th class has $a_i$ students and will make $b_i$ cups of milk tea.

Roundgod wants more students to savor milk tea, so he stipulates that every student can taste at most one cup of milk tea. Moreover, a student can't drink a cup of milk tea made by his class. The problem is, what is the maximum number of students who can drink milk tea?

The first line of input consists of a single integer $T$ $(1 \leq T \leq 25)$, denoting the number of test cases.

Each test case starts with a line of a single integer $n$ $(1 \leq n \leq 10^6)$, the number of classes. For the next $n$ lines, each containing two integers $a, b$ $(0 \leq a, b \leq 10^9)$, denoting the number of students of the class and the number of cups of milk tea made by this class, respectively.

It is guaranteed that the sum of $n$ over all test cases does not exceed $6 \times 10^6$.

For each test case, print the answer as a single integer in one line.