View Compilation Error

0_0_37535729_20042.cpp:5:14: error: 'const double exp' redeclared as different kind of symbol
 const double exp = 1e-9;
              ^
In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/cmath:44:0,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/x86_64-w64-mingw32/bits/stdc++.h:41,
                 from 0_0_37535729_20042.cpp:1:
GCC4.9.2/x86_64-w64-mingw32/include/math.h:148:18: note: previous declaration 'double exp(double)'
   double __cdecl exp(double _X);
                  ^
0_0_37535729_20042.cpp: In function 'int main()':
0_0_37535729_20042.cpp:36:18: error: invalid operands of types 'double' and '<unresolved overloaded function type>' to binary 'operator>'
   while (r - l > exp) 
                  ^