/in/foo.cc: In function 'int main()':
/in/foo.cc:10:41: warning: comparison with string literal results in unspecified behaviour [-Waddress]
 for(int i=1;i<=len;i++) {if(a[len+1-i]=="1") sum+=pow(2,i-1);}
                                         ^
/in/foo.cc:10:41: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
/in/foo.cc:9:40: warning: unused variable 'i' [-Wunused-variable]
 double len=a.size();unsigned long long i=1;double sum=0;
                                        ^