from functools import reduce

_dp={}
_dp_sum={}
M=10**9+7
def _build_dp_sum(arr):
    global _dp_sum
    _dp_partitial_sums=[[0 for _ in range(len(arr))] for _ in range(len(arr))]
    for i in range(len(arr)):
        _dp_partitial_sums[i][i]=arr[i]
    for i in range(len(arr)):
        for j in reversed(range(i)):
            _dp_partitial_sums[j][i]=_dp_partitial_sums[j+1][i]+arr[j]
    _dp_sum=[0]*len(arr)
    _dp_tmp=[0]*len(arr)
    for i in range(1, len(arr)):
        for j in range(i):
            print("tmp sum", i, j, i-j,_dp_partitial_sums[j+1][i])
            _dp_tmp[i]+=(i-j)*_dp_partitial_sums[j+1][i]
    _dp_sum[0]=0
    for i in range(1, len(arr)):
        _dp_sum[i]+=_dp_sum[i]
        for j in range(1, i):
            _dp_sum[i]+=_dp_partitial_sums[j][i]
        _dp_sum[i]+=arr[i]*sum(range(1, i))
        _dp_sum[i]+=_dp_partitial_sums[i][i]
    print(_dp_sum)
    print(_dp_tmp)
    print(_dp_partitial_sums)
    

def _next_sum(arr):
    global _dp
    global _dp_sum
    _dp=[0]*len(arr)
    _dp[0]=arr[0]
    _build_dp_sum(arr)
    for i in range(1, len(arr)):
        _dp[i]=reduce(lambda s,j: s+_dp[j], range(i), 0)+_dp_sum[i]
        print(f"_dp {_dp}, _dp_sum[i] {_dp_sum[i]}, _dp[i] {_dp[i]}")
    return _dp[-1]
       
            
def summingPieces(arr):
    _sum=_next_sum(arr)
    return _sum 
    # Write your code here

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    arr_count = int(input().strip())

    result = summingPieces(arr)

    fptr.write(str(result) + '\n')

    fptr.close()

    _dp_sum[0]=0
    for i in range(1, len(arr)):
        _dp_sum[i]+=_dp_sum[i-1]
        for j in range(1, i):
            _dp_sum[i]+=_dp_partitial_sums[j][i]
        _dp_sum[i]+=arr[i]*sum(range(2, i+1))
        _dp_sum[i]+=_dp_partitial_sums[i][i]