/in/foo.cc: In function 'int main()':
/in/foo.cc:12:29: error: 'abs' was not declared in this scope
    int dis = 2 * abs(i - len) - 1;
                             ^
/in/foo.cc:12:29: note: suggested alternative:
In file included from /in/foo.cc:2:0:
/usr/include/c++/6/cmath:103:5: note:   'std::abs'
     abs(_Tp __x)
     ^~~
/in/foo.cc:32:29: error: 'abs' was not declared in this scope
    int dis = 2 * abs(i - len) - 1;
                             ^
/in/foo.cc:32:29: note: suggested alternative:
In file included from /in/foo.cc:2:0:
/usr/include/c++/6/cmath:103:5: note:   'std::abs'
     abs(_Tp __x)
     ^~~