Problem solving in c++?

Question

C++ / C#

Hello, I have programmed a simple problem, in the solution of which it is necessary to use the declaration of a static variable:
Write a function with an integer parameter m, the result of which is equal to the sum of the parameters of the last three calls to this function.
The result is this code:

#include <iostream>
#include <cstdlib>

using namespace std;

void greeting(int n) {
    static int i = 1, s = 0, a = 1, b = 2, c = 3, a1 = 0, b1 = 0, c1 = 0;

    if (i == a) {
        a1 = n;
        a += 3;
    }
    else if (i == b) {
        b1 = n;
        b += 3;
    }
    else if (i == c) {
        c1 = n;
        c += 3;
    }

    i++;

    if (a1 > 0 && b1 > 0 && c1 > 0) {
        s = a1 + b1 + c1;
        cout << s << endl;
    }
    else {
        cout << s << endl;
    }
}
int main() {
       for (int i = 10; i <= 100; i += 10) {
           greeting(i);
       }
    return 0;
}

But something tells me that it could have been done much more beautifully and easier. Help with other better solutions.