foo.cpp: In function 'int main()':
foo.cpp:32:29: error: 'stdin' was not declared in this scope
     freopen("input.txt","r",stdin);
                             ^
foo.cpp:32:34: error: 'freopen' was not declared in this scope
     freopen("input.txt","r",stdin);
                                  ^
foo.cpp:33:30: error: 'stdout' was not declared in this scope
     freopen("output.txt","w",stdout);
                              ^
foo.cpp:35:21: error: 'scanf' was not declared in this scope
     scanf("%d",&s[0]);
                     ^
foo.cpp:42:31: error: 'printf' was not declared in this scope
     printf("%.5lf\n",maxarea/2);
                               ^