import numpy as np

N, K = map(int, input().split())
A = input()  # 使わない
MOD = 10 ** 9 + 7
dp = np.zeros(K + 1, dtype=np.int64)
dp[0] = 1
ans = 0
for i in range(N):
    acc = np.cumsum(dp) % MOD
    dp = acc.copy()
    dp[i + 1:] -= acc[:-(i + 1)]
    dp %= MOD
print(dp.sum() % MOD)