View Compilation Error

0_0_29643047_21944.cpp: In function 'int main()':
0_0_29643047_21944.cpp:21:61: error: 'flag' was not declared in this scope
     if(a[i]+a[j]<=1000 && book0[a[i]+a[j]] &&num[a[i]+a[j]]>flag){
                                                             ^