Pascal :

var
 a:array[1..50000,1..2] of longint;  
 dao,yuan,ci,cha,bo,l:array[0..50000] of longint;  
 max,i,j,n,t:longint;  
 m:boolean;  
procedure init;  
begin  
 readln(n);  
 for i:=1 to n do  
 begin  
  read(a[i,1],a[i,2]);  
  yuan[i]:=i;  
  dao[i]:=n+1-i;  
 end;  
end;  
procedure combine;  
begin  
 l[1]:=1;  
 j:=1;  
 bo[1]:=1;  
 repeat  
  if j<>1 then  
   if (a[l[j],1]<>l[j-1])and(a[l[j],2]<>l[j-1]) then  
   begin  
    writeln(-1);  
    halt;  
   end;  
  for t:=1 to 2 do  
   if bo[a[l[j],t]]=0 then  
   begin  
    bo[a[l[j],t]]:=1;  
    inc(j);  
    l[j]:=a[l[j-1],t];  
    break;  
   end;  
 until j=n;  
 m:=true;  
 if(a[l[n],1]=l[n-1])and(a[l[n],2]=l[1]) then m:=false;  
 if(a[l[n],2]=l[n-1])and(a[l[n],1]=l[1]) then m:=false;  
 if m then  
 begin  
  writeln(-1);  
  halt;  
 end;  
end;  
procedure judge;  
begin  
 max:=0;  
 for i:=1 to n do  
 begin  
  cha[i]:=yuan[l[i]]-i;  
 if cha[i]<0 then cha[i]:=cha[i]+n;  
  inc(ci[cha[i]]);  
end;  
 for i:=0 to n do  
  if ci[i]>max then max:=ci[i];  
 fillchar(ci,sizeof(ci),0);  
 for i:=1 to n do  
 begin  
  cha[i]:=dao[l[i]]-i;  
  if cha[i]<0 then cha[i]:=cha[i]+n;  
  inc(ci[cha[i]]);  
 end;  
 for i:=0 to n do  
  if ci[i]>max then max:=ci[i];  
end;  
begin  
 init;  
 combine;  
 judge;  
 writeln(n-max);  
end.