View Compilation Error

0_0_36945049_19990.cpp: In function 'int work()':
0_0_36945049_19990.cpp:36:75: error: expected primary-expression before '>=' token
            while(head<tail&&(Y(q[tail])-Y(q[tail-1]))*(X(j)-X(q[tail-1])))>=((Y(j)-Y(q[tail-1]))*(X(q[tail])-X(q[tail-1]))) --tail;
                                                                           ^
0_0_36945049_19990.cpp:36:125: error: lvalue required as decrement operand
            while(head<tail&&(Y(q[tail])-Y(q[tail-1]))*(X(j)-X(q[tail-1])))>=((Y(j)-Y(q[tail-1]))*(X(q[tail])-X(q[tail-1]))) --tail;
                                                                                                                             ^