Different Subsets For All Tuples

有一个长度为$n$的数列，每个位置上数字的值在$[1,m]$范围内，则共有$m^n$种可能的数列。分别求出每个数列中本质不同的子序列个数（包含空序列），然后求和，答案对$10^9+7$取模。（$1\le n,m\le10^6$）

For a sequence $ a $ of $ n $ integers between $ 1 $ and $ m $ , inclusive, denote $ f(a) $ as the number of distinct subsequences of $ a $ (including the empty subsequence). You are given two positive integers $ n $ and $ m $ . Let $ S $ be the set of all sequences of length $ n $ consisting of numbers from $ 1 $ to $ m $ . Compute the sum $ f(a) $ over all $ a $ in $ S $ modulo $ 10^{9}+7 $ .

The only line contains two integers $ n $ and $ m $ ( $ 1<=n,m<=10^{6} $ ) — the number of elements in arrays and the upper bound for elements.

Print the only integer $ c $ — the desired sum modulo $ 10^{9}+7 $ .

1 3

6

2 2

14

3 3

174