const mo=;

 var pow,f,exf:array[-..]of int64;

     n,k,i:longint;

     ans,x,y:int64;

     s,t:ansistring;

 function c(x,y:longint):int64;

 begin

  exit(f[x]*exf[y] mod mo*exf[x-y] mod mo);

 end;

 function min(x,y:int64):int64;

 begin

  if x<y then exit(x);

  exit(y);

 end;

 begin

  assign(input,'1.in'); reset(input);

  assign(output,'1.out'); rewrite(output);

  readln(n,k);

  readln(s);

  readln(t);

  pow[]:=;

  for i:= to n do pow[i]:=pow[i-]* mod mo;

  f[]:=;

  for i:= to n do f[i]:=f[i-]*i mod mo;

  exf[]:=; exf[]:=;

  for i:= to n do exf[i]:=exf[mo mod i]*(mo-mo div i) mod mo;

  for i:= to n do exf[i]:=exf[i-]*exf[i] mod mo;

  ans:=;

  for i:= to n do

  begin

   x:=ord(t[i])-ord('a'); y:=ord(s[i])-ord('a');

   ans:=(ans+min(x,y)*c(n-i,k-) mod mo*pow[k-] mod mo) mod mo;

   if s[i]<t[i] then

   begin

    ans:=(ans+c(n-i,k)*pow[k] mod mo) mod mo;

    ans:=(ans+(x-y-)*c(n-i,k-) mod mo*pow[k-] mod mo) mod mo;

   end;

   if s[i]<>t[i] then dec(k);

  end;

  writeln(ans);

  close(input);

  close(output);

 end.