The input is of multiple testcases, and each testcase starts with N, the length of the sequence, and K, the number of pairs of intervals. You only know when you stop after got N == -1.

Naive ideas will get TLE even for testcase #1, and you will see your code runs out of 2s limit when feeding a testcase with N=10^9 and K=5000, when all pairs are good and you should output 5000.