func_code_string: #include <bits/stdc++.h>   using namespace std;   const int N = 5e5 + 5;   char a[N], b[N];   int main() {     int n, k;     cin >> n >> k;     scanf( %s %s , a + 1, b + 1);     long long ans = 0, t = 1;     for (int i = 1; i <= n; ++i) {       t = t * 2;       if (b[i] ==  a ) t--;       if (a[i] ==  b ) t--;       if (t > k) t = k + 1;       ans += (t > k ? k : t);     }     printf( %lld n , ans);   }   