- Solution

// sum.cpp
#include <iostream>
template<typename ...Args>     // Without an initial value
auto sum(Args&& ... args){
    return ( ... + args);
}
template<typename T, typename ...Args>          // with an initial value    
auto sumWithStart(T&& t, Args&& ... args){
    return ( t + ... + args);
}
int main(){
  std::cout << sum(1, 2, 3) << std::endl;
  std::cout << sumWithStart(20, 1, 2, 3) << std::endl;
}