Cost of Solving

Description Chef has two integer variables sum=1 and cur=0. He can perform two operations with these variables $$\ O1: \text{ cur=sum, Where O1 has cost c1}$$ $$\ O2: \text{ sum=sum+cur, Where O2 has cost c2}$$ These two operations can be performed in any order and any number of times. For a given integer n , chef wants to find the minimal cost of all the operations so that, chef could get sum=n