program blast(input,output);
var
 dp:array[0..2000,0..2000]of longint;
 ch1,ch2:array[1..2000]of char;
 k,l,p:longint;
 i,j:longint;
 ch:char;

begin
 assign(input,'blast.in');
 reset(input);
 repeat
  read(ch);
  if ord(ch)=13 then break
  else
  begin
   inc(k);
   ch1[k]:=ch;
  end;
 until ch='';
 readln;
 repeat
  read(ch);
  if ord(ch)=13 then break
  else
  begin
   inc(l);
   ch2[l]:=ch;
  end;
 until ch='';
 readln(p);
 close(input);

 for i:=1 to k do
  dp[i,0]:=dp[i-1,0]+p;
 for j:=1 to l do
  dp[0,j]:=dp[0,j-1]+p;

 for i:=1 to k do
  for j:=1 to l do
  begin
   dp[i,j]:=dp[i-1,j-1]+abs(ord(ch1[i])-ord(ch2[j]));
   if dp[i-1,j]+p<dp[i,j] then dp[i,j]:=dp[i-1,j]+p;
   if dp[i,j-1]+p<dp[i,j] then dp[i,j]:=dp[i,j-1]+p;
  end;

 assign(output,'blast.out');
 rewrite(output);
 writeln(dp[k,l]);
 close(output);
end.