feat: do a main operator with user input

exercises/calculator.cpp

@@ -10,3 +10,81 @@
   Multiplication: 8
   Division: 2
 */
+
+#include <iostream>
+#include <string>
+
+using namespace std;
+
+const string ERROR_COLOR = "\033[31m";
+const string NORMAL_COLOR = "\033[0m";
+
+const int ZERO = 48;
+const int NINE = 57;
+const int MAX_DIGIT = 10;
+const int MINUS = 45;
+
+int insertNumber(string position);
+bool checkStringIsNumber(string in);
+
+float doSum(float n1, float n2);
+float doSub(float n1, float n2);
+float doMolt(float n1, float n2);
+float doDiv(float n1, float n2);
+
+void doOperation(float n1, float n2);
+
+int main(){
+  float num1 = insertNumber("first");
+  float num2 = insertNumber("second");
+
+  doOperation(num1, num2);
+  return EXIT_SUCCESS;
+}
+
+
+
+int insertNumber(string position){
+  string insert;
+  bool isCorrect = true;
+  do {
+    if(!isCorrect) cout << ERROR_COLOR+("[Insert a valid input] ")+NORMAL_COLOR;
+    cout << "Insert "<< position <<" number: ";
+    getline(cin, insert);
+
+    isCorrect = checkStringIsNumber(insert);
+  } while(!(isCorrect));
+
+  return stoi(insert);
+}
+
+bool checkStringIsNumber(string in){
+  if(in.empty()) return false;
+  if(in.size() > MAX_DIGIT) return false;
+
+  for(int i = 0; i < in.size(); i++)
+    if((in.at(i) < ZERO || in.at(i) > NINE) && in.at(i) != MINUS) 
+      return false;
+
+  return true;
+}
+
+float doSum(float n1, float n2) {
+  return n1+n2;
+}
+float doSub(float n1, float n2) {
+  return n1-n2;
+}
+float doMolt(float n1, float n2) {
+  return n1*n2;
+}
+float doDiv(float n1, float n2) {
+  return (n2 != 0) ? n1/n2 : -1;
+}
+
+void doOperation(float n1, float n2) {
+  cout << "SUM: " << doSum(n1, n2) << endl;
+  cout << "Difference: " << doSub(n1, n2) << endl;
+  cout << "Moltiplication: " << doMolt(n1, n2) << endl;
+  cout << "Division: " << doDiv(n1, n2) << endl;
+}