'stoi' was not declared in this scope [C++]

I've build this simple application in C++ using the GCC COMPILER

#include <iostream>
#include <string>

using namespace std;

int main()
{
string hello = "90";
int dunno;

dunno = stoi(hello);
cout << dunno << endl;
return 0;
}

But the compiler tells me that 'stoi' wasn't declared in this scope although I included string.h

Here is a screen of my compiler settings: http://ift.tt/1BmOaW8