Get Min Time (Earliest Time Usage ≥ Max)

Each user contributes 1 usage at every integer second from start to end. Find the integer second at which the cumulative usage first reaches max_usage. CSWork standard I/O convention: output −1 if the threshold is never reached.

输入格式

The first line contains n max_usage (1≤n≤100000, 1≤max_usage≤10¹⁴), followed by n lines of start end (1≤start≤end≤10⁹).

输出格式

Output a single integer answer.

样例 1

输入：

2 3
1 3
2 2

输出：

2

样例 2

输入：

1 1
5 5

输出：

5

样例 3

输入：

1 2
1 1

输出：

-1

样例说明

Example 1: the cumulative total is 1 at second 1; at second 2 both users contribute 1 each, for a cumulative 3, so output 2. Example 2: starting and ending in the same second still contributes 1, and the threshold is first reached at second 5. Example 3: the total usage is only 1 and never reaches 2, so output −1 per this site's convention.