func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long maxn = 5e5 + 5;   long long n, k;   long long ans, a, b;   char s[maxn], t[maxn];   int main() {     cin >> n >> k;     scanf( %s%s , s + 1, t + 1);     for (int i = 1; i <= n; ++i) {       a = a << 1 | (s[i] -  a );       b = b << 1 | (t[i] -  a );       if (b - a >= k) {         ans += (n - i + 1) * k;         break;       }       ans += (b - a + 1);     }     cout << ans << endl;     return 0;   }   