Hope this will helpful for someone.
Never repeat my mistake. Never start name of global variable with an underscore.
For example Accepted code in Codechef.
The same code gives Runtime error with addition:
#define where _end, i.e by renaming where variable to _end.
Again, the same code gives Wrong Answer with addition:
#define size _end, i.e by renaming another variable "size" to _end.
In my point of view, variables beginning with an underscore are reserved in the global namespaces.
Can somebody better explain what is happening here? I tried in some problems in Codeforces, but couldn't trigger it.