double calculate(double, double, double (*f)(double, double));

@@ -0,0 +1,35 @@
+#include <iostream>
+
+using std::cout;
+using std::cin;
+
+// this function compute harmonic average of two numbers
+double harmonicAverage(double, double);
+
+int main() {
+    int x, y;
+    double average;
+
+    cout << "Enter two numbers: ";
+    while (cin >> x >> y, x != 0 && y != 0) {
+        if (cin.fail()) {
+            cout << "Invalid error. Please, enter two numbers.\n";
+            cin.clear();
+            while (cin.get() != '\n');
+            continue;
+        }
+        average = harmonicAverage(x, y);
+        cout << "Harmonic average of " << x << " and " << y << " is " << average;
+
+        cout << "\nEnter next two numbers: ";
+    }
+
+    cin.get();
+    cin.get();
+
+    return 0;
+}
+
+double harmonicAverage(double x, double y) {
+    return 2.0 * x * y / (x + y);
+}