Let $k>1$ be a given positive integer. A set $S$ of positive integers is called [i]good[/i] if we can colour the set of positive integers in $k$ colours such that each integer of $S$ cannot be represented as sum of two positive integers of the same colour. Find the greatest $t$ such that the set $S=\{a+1,a+2,\ldots ,a+t\}$ is [i]good[/i] for all positive integers $a$.

[i]A. Ivanov, E. Kolev[/i]
To solve this problem, we need to determine the largest integer \( t \) such that the set \( S = \{a+1, a+2, \ldots, a+t\} \) is *good* for all positive integers \( a \). A set \( S \) is *good* if we can color the set of positive integers in \( k \) colors such that no element of \( S \) can be represented as the sum of two positive integers of the same color.

1. **Understanding the Problem:**
   - We need to color the integers in \( k \) colors.
   - For the set \( S = \{a+1, a+2, \ldots, a+t\} \) to be *good*, no element in \( S \) should be the sum of two integers of the same color.

2. **Initial Consideration:**
   - Consider the smallest possible \( a \), which is \( a = 1 \).
   - The set \( S \) becomes \( \{2, 3, \ldots, t+1\} \).

3. **Coloring Strategy:**
   - We need to ensure that no element in \( S \) can be written as the sum of two integers of the same color.
   - If we color the integers \( 1, 2, \ldots, t+1 \) in \( k \) colors, we need to avoid any two integers of the same color summing to an element in \( S \).

4. **Maximizing \( t \):**
   - The maximum \( t \) is determined by the constraint that no two integers of the same color can sum to an element in \( S \).
   - If we use \( k \) colors, the worst-case scenario is that each color is used at least twice within the range \( \{1, 2, \ldots, t+1\} \).

5. **Finding the Maximum \( t \):**
   - Consider the set \( \{1, 2, \ldots, k+1\} \). If we color these \( k+1 \) integers with \( k \) colors, by the pigeonhole principle, at least one color will be used twice.
   - The sum of the two integers of the same color will be at most \( 2(k+1) \).
   - Therefore, the set \( \{2, 3, \ldots, 2k+1\} \) can be colored such that no element is the sum of two integers of the same color.

6. **Conclusion:**
   - The largest \( t \) such that \( S = \{a+1, a+2, \ldots, a+t\} \) is *good* for all positive integers \( a \) is \( 2k-1 \).

The final answer is \( \boxed{2k-1} \).