Answer 1

this server the purpose. it would be better if you include the condition inside main.
#include <iostream>
using namespace std;
int add_alternate_numbers(int a,int b)
{
int sum=0;
if(b>=a)
{
for(a;a<=b;a=a+2)
{
sum=sum+a;
}
return sum;
}
else
{
return -1;
}
}
int main() {
int a,b,result;
cout<<"Enter the range to claculate sum(smaller number followed by larger one):"<<endl;
cin>>a>>b;
result=add_alternate_numbers(a,b);
if(result==-1)
{
cout<"Invalid input values";
}
else
{
cout<<endl<<"Resultant="<<result;
}
return 0;
}