Move Project Euler under puzzles.

puzzles/project_euler/p006_SumSquareDiff.cpp

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+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+*/
+#include <iostream>
+
+/*
+Sum square difference
+Problem 6
+
+The sum of the squares of the first ten natural numbers is,
+
+1^2 + 2^2 + ... + 10^2 = 385
+The square of the sum of the first ten natural numbers is,
+
+(1 + 2 + ... + 10)^2 = 55^2 = 3025
+Hence the difference between the sum of the squares of the first ten natural
+numbers and the square of the sum is 3025 - 385 = 2640.
+
+Find the difference between the sum of the squares of the first one hundred
+natural numbers and the square of the sum.
+
+Solution:
+25164150
+ */
+
+int main ( void )
+{
+	const long long n = 100;
+	long long s = n*(n+1)/2;
+	s*=s;
+	for ( long long i = 1; i <= n; ++i )
+		s-=i*i;
+	std::cout << s << std::endl;
+
+	return 0;
+}
+