```python CST = 10 ** 9 + 7 n = int(input().strip()) s = input().strip() s = s[-1::-1] initRcount = s.count('R') ns = [0] * n ms = [0] * n ns[0] = 1 k = 1 if s[0] == 'R': ms[0] = 1 else: ms[0] = 2 for i in range(1, n): if s[i] == 'B': ns[i] = (0 * ms[i - 1] + ns[i - 1]) + (1 * 2 ** i + k) ms[i] = ms[i - 1] + 2 ** i else: ns[i] = (1 * ms[i - 1] + ns[i - 1]) ms[i] = ms[i - 1] k = 2 * k + 2 ** i print(ns[n - 1] % CST) ```