codeforces

n, k = map(int, input().split())
a = input()
b = input()
res = 0
ans = 0
for i in range(0, n):
    res = min(res * 2 + (b[i] == 'b') - (a[i] == 'b'), k)
    ans += min(res + 1, k)
print(ans)