sys is a two-dimensional array. sys[g][nun] holds some value which is assigned to rt in line 5.
continue will break-out of the for-loop. That one should have been easy to look-up in any begining C++ book. Or, online... cppreference.com is a good place to look-up keywords and standard library functions.
Your book should have -= too. Or, at least +=.
x -=2 is the same as x = x - 2.
You might need to study arrays and pointers too. (And, pointers to arrays.)
