int sum(int a, int b)
{
  if(a > b) 
  {
    return 0;
  }
  else 
  {
    return a + sum(a+1, b);
  }
}
    
    
Thought process.
If a > b, then there are no numbers that are greater than or equal to a and less than or equal to b. The sum of no numbers is 0.
If a <= b, then
| sum(a,b) | = a + (a+1) + ... + b | 
| = a + sum(a+1, b) |