Approach for problem

Can anyone suggest the approach for the following CSES problem

An easy approach is to calculate the probability of getting sum equal to j if you have thrown the dice i times, dp[i][j], where i goes from 1 to n and j from 1 to 6n. That let you build a table and then answer is the sum: dp[n][a] + dp[n][a+1] + .... + dp[n][b]

Thanks @carre