Users need to specify problem/contest identifier to acmX for processing. This is an specification of what is every platform id.
In codeforces from the URL of a problem you can identify both contest id and problem id in the following format such that PROBLEM_ID=CONTEST_ID-LETTER
:
https://codeforces.com/contest/{CONTEST_ID}/problem/{LETTER}
For example in problem (https://codeforces.com/contest/1101/problem/G) identifiers are CONTEST_ID=1101
and PROBLEM_ID=1101-G
.
Notice that contest id don't coincide with round number.