Hi @Shaolys Welcome to the forum! 
Just out of curiosity, I asked our AI Writer the same question and got an answer very similar to @Ashwin_Mittal1's answer. One advantage of this solution is that it's broken down into different parts (valid_res2, valid_res3, valid_res4), so you can easily change each part independently without affecting the other parts:
{formtext: name=sum1}
{formtext: name=var1; default=49}
{formtoggle: name=var2; default=no}- {formtext: name=res2; default=10}{endformtoggle}
{formtoggle: name=var3; default=no}- {formtext: name=res3; default=5}{endformtoggle}
{formtoggle: name=var4; default=no}- {formtext: name=res4; default=18}{endformtoggle}
{valid_res2=res2 if var2 else 0}
{valid_res3=res3 if var3 else 0}
{valid_res4=res4 if var4 else 0}
Total {=sum1+var1+valid_res2+valid_res3+valid_res4}