C# equivalent of C++ map<string,double>

I want to keep some totals for different accounts. In C++ I'd use STL like this:

map<string,double> accounts;

// Add some amounts to some accounts.
accounts["Fred"] += 4.56;
accounts["George"] += 1.00;
accounts["Fred"] += 1.00;

cout << "Fred owes me $" << accounts['Fred'] << endl;

Now, how would I do the same thing in C# ?