View Compilation Error

0_0_13100292_24028.cpp:4:1: error: 'map' does not name a type
 map<int,map<int,__int64> >b;
 ^
0_0_13100292_24028.cpp: In function 'int main()':
0_0_13100292_24028.cpp:26:3: error: 'b' was not declared in this scope
   b[0][0]=a[0];
   ^