#include <bits/stdc++.h>   const int N = 500000;   int n, k;   char s[N + 1], t[N + 1];   int main() {     scanf( %d%d%s%s , &n, &k, s, t);     int r = 0;     long long ans = 0;     for (int i = 0; i < n; ++i) {       if (r < k) {         r *= 2;         r += t[i] - s[i];       }       r = std::min(r, k);       ans += r == k ? r : r + 1;     }     printf( %lld n , ans);     return 0;   }   