Входная строка имела неверный формат - C# (186719)

 while (true)
            {
                Thread.Sleep(1);
                Random rand = new Random(DateTime.Now.Millisecond);
                Int64 a = rand.Next(2, 999);
                Int64 b = rand.Next(2, 999);
                Int64 c = rand.Next(2, 999);
                Int64 d = rand.Next(2, 999);
                String A = Convert.ToString(a);
                String B = Convert.ToString(b);
                String C = Convert.ToString(c);
                String D = Convert.ToString(d);
                Int64 result = Convert.ToInt64(A) * (4 * Convert.ToInt64(B)) * Convert.ToInt64(C) * (Convert.ToInt64(D) * Convert.ToInt64(D));
                String Resultat = A + "x(4x" + B + ")x" + C + "x" + "(" + D + "x" + D + ")" + "=" + result;
                Int64 aa = rand.Next(2, 999);
                Int64 bb = rand.Next(2, 999);
                Int64 cc = rand.Next(2, 999);
                String AA = Convert.ToString(aa);
                String BB = Convert.ToString(bb);
                String CC = Convert.ToString(cc);
                Int64 DD = Convert.ToInt64(BB) * Convert.ToInt64(BB) - 4 * Convert.ToInt64(AA) * Convert.ToInt64(CC);
                String DDResult1 = Convert.ToString(DD);
                double Root = Convert.ToDouble(DDResult1);
                Root = Math.Sqrt(Root);
                String DDRoot = Root.ToString();
                String DR1 = AA + "Yx" + AA + "Y+" + BB + "+" + CC + "=0";
                String DR2 = "Square root D=" + DDRoot;
                Int64 RR1 = -Convert.ToInt64(BB) + Convert.ToInt64(DDRoot) / 2 * Convert.ToInt64(AA);
                Int64 RR2 = -Convert.ToInt64(BB) - Convert.ToInt64(DDRoot) / 2 * Convert.ToInt64(AA);
                String RR1S = Convert.ToString(RR1);
                String RR2S = Convert.ToString(RR2);
                StreamWriter sw1 = new StreamWriter("output.math", true);
                sw1.WriteLine(Resultat);
                sw1.WriteLine("D=" + DDResult1);
                sw1.WriteLine(DR1);
                sw1.WriteLine(DR2);
                sw1.WriteLine("Y1=" + RR1S);
                sw1.WriteLine("Y2=" + RR2S);
                sw1.Close();
            }

 Int64 RR1 = -Convert.ToInt64(BB) + Convert.ToInt64(DDRoot) / 2 * Convert.ToInt64(AA);
                Int64 RR2 = -Convert.ToInt64(BB) - Convert.ToInt64(DDRoot) / 2 * Convert.ToInt64(AA);