solve problem 6

Program.cs

@@ -110,8 +110,18 @@ namespace Euler {
   		}
   	}
 
+    static long Problem6() {
+    	long sum = 0;
+      long sumSq = 0;
+    	for (long i = 1; i <= 100; i++) {
+    		sum += i;
+    		sumSq += i * i;
+    	}
+    	return sum * sum - sumSq;
+  	}
+
     static void Main(string[] args) {
-      WriteLine(Problem5());
+      WriteLine(Problem6());
     }
   }
 }