View Compilation Error

0_0_22171629_7903.cpp: In function 'int main()':
0_0_22171629_7903.cpp:12:63: error: expected ';' before ')' token
       if(((2*a1+i-1)*i)==2*n)&&a1>0&&(i%2==0||(2*a1+i-1)%2==0))
                                                               ^
0_0_22171629_7903.cpp:12:32: error: label 'a1' used but not defined
       if(((2*a1+i-1)*i)==2*n)&&a1>0&&(i%2==0||(2*a1+i-1)%2==0))
                                ^