rec.cpp

dp[i][j] = no. of ways to get j in the first i slots
dp[0][j] = 0, dp[0][0] = 1
dp[i][j] = sum[i - 1][b] - sum[(k = a to b)
	
sum[i - 1][j] = sum dp[i - 1][k] k = 1 to j


dp[i]
dp[i][j] = no. of ways to partition i into j partitions
dp[i][j] = dp[i - 1][j - 1] + j*dp[i - 1][j]

sum j*dp[i][j] = 
sum dp[i][j] = sum dp[i - 1][j] (j = 0 to inf) + sum j*dp[i - 1][j] (j = 0 to inf)
			 = sum (j+1) dp[i - 1][j] (j = 0 to inf)
	

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