n, l = map(int, input().split())
mod = 10**9 + 7

if n < l:
    print(pow(2, n - 1, mod))
    exit()

DP = [0 for _ in range(n + 1)]
DP[0] = 1
for i in range(1, l):
    DP[i] = pow(2, i - 1, mod)
s = 0
for i in range(l):
    s += DP[i]
    s %= mod
for i in range(l, n + 1):
    DP[i] = s
    s -= DP[i - l]
    s += DP[i]
    s %= mod
print(DP[n])