How to recursively calculate the sum of numbers from a to b?

Question

C++ / C#

You need to recursively determine the sum of numbers in the range from a to b. I did it in a loop, but I need to do it recursively.
Here's what happened. I can't figure out what to write in the main method to make it work.

#include <iostream>
using namespace std;
int  summa(int a, int b)  
   {
      if (a==b) return b;
      else a+summa(b,a);
   }
int main()
{
   int a,b;
   cin>>a>>b;
   cout<<summa(a,b);
  return 0;
}

Answer 1

https://ideone.com/VXfoqI

Answer 2

int sum(int a, int b) {
  return a == b? b : a + sum(a + 1, b);
}

But it would be worth learning Russian.