#include <bits/stdc++.h>
using namespace std;
const int P = 1e9 + 7, INF = 0x3f3f3f3f;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long qpow(long long a, long long n) {
  long long r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1) r = r * a % P;
  return r;
}
long long inv(long long first) {
  return first <= 1 ? 1 : inv(P % first) * (P - P / first) % P;
}
inline int rd() {
  int first = 0;
  char p = getchar();
  while (p < '0' || p > '9') p = getchar();
  while (p >= '0' && p <= '9') first = first * 10 + p - '0', p = getchar();
  return first;
}
const int N = 1e6 + 10;
int n;
long long k;
char s[N], t[N];
int main() {
  scanf("%d%lld%s%s", &n, &k, s + 1, t + 1);
  long long ans = 0, tot = 1, INF = 1e15;
  for (int i = 1; i <= n; ++i) {
    tot <<= 1;
    if (s[i] == 'b') --tot;
    if (t[i] == 'a') --tot;
    ans += min(k, tot);
    tot = min(tot, INF);
  }
  printf("%lld\n", ans);
}
