#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define pb push_back
#define vi vector<int>
#define vl vector<ll>
#define pi pair<int, int>
#define pl pair<ll,ll>
#define all(x) (x).begin(),(x).end()
const int maxn=1e5+10;
vi to(maxn);
vector<vi> in(maxn);
vi seen(maxn,0);
int dfs(int cur) {
seen[cur]=2;
if (seen[to[cur]]==2) {
return to[cur];
}
return dfs(to[cur]);
}
vector<vi> dp(maxn,vi(2,0));
void dfs2(int cur) {
int cnt=0;
for (auto to:in[cur]) {
if (seen[to]==3) {
continue;
}
dfs2(to);
cnt+=min(dp[to][0],dp[to][1]+1);
}
dp[cur][1]=cnt;
dp[cur][0]=cnt+1;
for (auto to:in[cur]) {
if (seen[to]==3) {
continue;
}
cnt-=min(dp[to][0],dp[to][1]+1);
dp[cur][0]=min(dp[cur][0],cnt+min(dp[to][0],dp[to][1])+1);
cnt+=min(dp[to][0],dp[to][1]+1);
}
}
void dfsset1(int cur) {
seen[cur]=1;
for (auto to:in[cur]) {
if (seen[to]==1) {
continue;
}
dfsset1(to);
}
}
int32_t main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
if (n%2) {
cout << -1 << '\n';
return 0;
}
map<string,int> gt;
string s,ss;
int a,b;
int t=0;
int ans=0;
for (int i=0; i<n; i++) {
cin >> s >> ss;
if (!gt.count(s)) {
gt[s]=t++;
}
a=gt[s];
if (!gt.count(ss)) {
gt[ss]=t++;
}
b=gt[ss];
to[a]=b;
in[b].pb(a);
}
for (int i=0; i<n; i++) {
if (seen[i]) {
continue;
}
int a=dfs(i);
if (to[a]==a) {
seen[a]=3;
dfs2(a);
ans+=min(dp[a][0],dp[a][1]+1);
dfsset1(a);
}
else if (to[to[a]]==a) {
seen[a]=3;
seen[to[a]]=3;
dfs2(a);
dfs2(to[a]);
ans+=min(dp[a][1]+dp[to[a]][1],min(dp[a][0],dp[a][1]+1)+min(dp[to[a]][0],dp[to[a]][1]+1));
dfsset1(a);
}
else {
int mn=1e9;
int temp=to[to[a]];
to[to[a]]=a;
seen[a]=3;
seen[to[a]]=3;
dfs2(a);
dfs2(to[a]);
mn=min(dp[a][1]+dp[to[a]][1]+1,min(dp[a][0],dp[a][1]+1)+min(dp[to[a]][0],dp[to[a]][1]+1));
seen[a]=0;
seen[to[a]]=0;
to[to[a]]=temp;
a=to[a];
to[to[a]]=a;
seen[a]=3;
seen[to[a]]=3;
dfs2(a);
dfs2(to[a]);
mn=min(mn,min(dp[a][1]+dp[to[a]][1]+1,min(dp[a][0],dp[a][1]+1)+min(dp[to[a]][0],dp[to[a]][1]+1)));
ans+=mn;
dfsset1(a);
}
}
cout << ans << '\n';
return 0;
}
