View Compilation Error

0_0_21695582_23882.cpp: In function 'int build(int, int, int)':
0_0_21695582_23882.cpp:34:21: error: 'a1' was not declared in this scope
   a[mid].minn=min(a[a1].minn,a[a2].minn);
                     ^
0_0_21695582_23882.cpp:34:32: error: 'a2' was not declared in this scope
   a[mid].minn=min(a[a1].minn,a[a2].minn);
                                ^