foo.cpp: In function 'int main()':
foo.cpp:6:20: error: 'a' was not declared in this scope
  scanf("%d%d%d%d",&a,&b,&c,&d);
                    ^
foo.cpp:6:23: error: 'b' was not declared in this scope
  scanf("%d%d%d%d",&a,&b,&c,&d);
                       ^
foo.cpp:6:26: error: 'c' was not declared in this scope
  scanf("%d%d%d%d",&a,&b,&c,&d);
                          ^
foo.cpp:6:29: error: 'd' was not declared in this scope
  scanf("%d%d%d%d",&a,&b,&c,&d);
                             ^