View Compilation Error

0_0_24248479_2261.cpp: In function 'int main()':
0_0_24248479_2261.cpp:31:21: error: call of overloaded 'stoi(std::string&)' is ambiguous
             t=stoi(a);
                     ^
0_0_24248479_2261.cpp:31:21: note: candidates are:
0_0_24248479_2261.cpp:6:4: note: ll stoi(std::string)
 ll stoi(string a) {
    ^
In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/string:52:0,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/locale_classes.h:40,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/ios_base.h:41,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/ios:42,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/ostream:38,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/iostream:39,
                 from 0_0_24248479_2261.cpp:1:
GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/basic_string.h:2858:3: note: int std::stoi(const string&, std::size_t*, int)
   stoi(const string& __str, size_t* __idx = 0, int __base = 10)
   ^