n, m = map(int, input().split())
positions = list(map(int, input().split()))
current_sum = 0
modulo = 1000000007
for index in range(1, m + 1):
    current_sum += positions[index - 1]
    current_sum %= modulo
    if positions[index]:
        positions[index] = current_sum
for index in range(m + 1, n):
    current_sum -= positions[index - m - 1]
    current_sum += positions[index - 1]
    current_sum %= modulo
    if positions[index]:
        positions[index] = current_sum
print(positions[-1])