use proconio::input; fn main() { input! { n:usize, a:[usize;n], } let comb = Combination::::new(n - 1); let mut ans = ModInt::::new(0); for i in 0..n { ans += ModInt::new(a[i]) * comb.comb(n - 1, i); } println!("{}", ans); } const MOD: usize = 1000000007; use combination::*; use modint::*; mod combination { use crate::modint::*; pub struct Combination { fac: Vec>, finv: Vec>, } impl Combination { pub fn new(n: usize) -> Self { let mut fac = vec![ModInt::new(1); n + 1]; let mut inv = vec![ModInt::new(1); n + 1]; let mut finv = vec![ModInt::new(1); n + 1]; for i in 2..=n { fac[i] = fac[i - 1] * ModInt::::new(i); inv[i] = -inv[MOD % i] * ModInt::::new(MOD / i); finv[i] = finv[i - 1] * inv[i]; } Self { fac, finv } } pub fn comb(&self, n: usize, k: usize) -> ModInt { if n < k { ModInt::new(0) } else { self.fac[n] * self.finv[n - k] * self.finv[k] } } } } mod modint { use std::fmt; use std::ops; #[derive(Copy, Clone, PartialEq, Eq)] pub struct ModInt { pub val: usize, } impl ModInt { pub fn new(val: usize) -> Self { Self { val: val % MOD } } pub fn pow(mut self, mut e: usize) -> Self { let mut res = Self::new(1); while 0 < e { if e & 1 != 0 { res *= self; } self *= self; e >>= 1; } res } } impl From for ModInt { fn from(value: usize) -> Self { Self { val: value % MOD } } } impl fmt::Display for ModInt { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "{}", self.val) } } impl fmt::Debug for ModInt { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "{}", self.val) } } impl ops::Neg for ModInt { type Output = Self; fn neg(self) -> Self::Output { Self { val: (MOD - self.val) % MOD, } } } impl ops::Add for ModInt { type Output = Self; fn add(self, rhs: Self) -> Self::Output { Self { val: (self.val + rhs.val) % MOD, } } } impl ops::AddAssign for ModInt { fn add_assign(&mut self, rhs: Self) { *self = *self + rhs; } } impl ops::Mul for ModInt { type Output = Self; fn mul(self, rhs: Self) -> Self::Output { Self { val: self.val * rhs.val % MOD, } } } impl ops::MulAssign for ModInt { fn mul_assign(&mut self, rhs: Self) { *self = *self * rhs; } } impl ops::Sub for ModInt { type Output = Self; fn sub(mut self, rhs: Self) -> Self::Output { if self.val < rhs.val { self.val += MOD; } Self { val: (self.val - rhs.val) % MOD, } } } impl ops::SubAssign for ModInt { fn sub_assign(&mut self, rhs: Self) { if self.val < rhs.val { self.val += MOD; } *self = *self - rhs; } } impl ops::Div for ModInt { type Output = Self; fn div(self, rhs: Self) -> Self { assert!(rhs.val != 0); self * rhs.pow(MOD - 2) } } impl ops::DivAssign for ModInt { fn div_assign(&mut self, rhs: Self) { *self = *self / rhs } } }