/in/foo.cc: In function 'int main()':
/in/foo.cc:36:5: error: 'first' was not declared in this scope
     first = (a[0].y+1)*2+1;
     ^~~~~
/in/foo.cc:44:13: warning: unused variable 'dx' [-Wunused-variable]
         int dx = abs(a[i-1].x - a[i].x);
             ^~
/in/foo.cc:45:13: warning: unused variable 'dy' [-Wunused-variable]
         int dy = abs(a[i-1].y - a[i].y);
             ^~