View Compilation Error

0_0_33534153_17103.cpp: In function 'int gcd(int, int)':
0_0_33534153_17103.cpp:8:10: error: assignment of read-only parameter 'a'
         a+=b;
          ^
0_0_33534153_17103.cpp:9:10: error: assignment of read-only parameter 'b'
         b=a-b;
          ^
0_0_33534153_17103.cpp:10:10: error: assignment of read-only parameter 'a'
         a=a-b;
          ^