View Compilation Error

0_0_18784246_30771.cpp: In function 'int main()':
0_0_18784246_30771.cpp:72:3: error: 'thread' was not declared in this scope
   thread a1(th, 1);
   ^
0_0_18784246_30771.cpp:73:10: error: expected ';' before 'a2'
   thread a2(th, 2);
          ^
0_0_18784246_30771.cpp:74:10: error: expected ';' before 'a3'
   thread a3(th, 3);
          ^
0_0_18784246_30771.cpp:75:10: error: expected ';' before 'a4'
   thread a4(th, 4);
          ^
0_0_18784246_30771.cpp:76:3: error: 'a1' was not declared in this scope
   a1.join();
   ^
0_0_18784246_30771.cpp:77:3: error: 'a2' was not declared in this scope
   a2.join();
   ^
0_0_18784246_30771.cpp:78:3: error: 'a3' was not declared in this scope
   a3.join();
   ^
0_0_18784246_30771.cpp:79:3: error: 'a4' was not declared in this scope
   a4.join();
   ^