string str = project.Variables["Text"].Value;
str = str
.Replace("Сколькобудет", "").Replace("? Введите число ниже!", "")
.Replace("один", "1")
.Replace("два", "2")
.Replace("три", "3")
.Replace("четыре", "4")
.Replace("пять", "5")
.Replace("шесть", "6")
.Replace("восемь", "8")
.Replace("семь", "7")
.Replace("девять", "9")
.Replace("десять", "10")
.Replace("иплюс", "+").Replace("плюс", "+")
.Replace("иминус", "-").Replace("минус", "-");
string s = string.Join(" ", (from Match m in Regex.Matches(str, @"\D") select m.Value));
int[] i = str.Split(new char[] {'+', '-'}).Select(int.Parse).ToArray();
switch (s) {
case "+ +": return i[0] + i[1] + i[2];
case "+ -": return i[0] + i[1] - i[2];
case "- +": return i[0] - i[1] + i[2];
case "- -": return i[0] - i[1] - i[2];
}