#include <bits/stdc++.h>
using namespace std;
#define X first
#define Y second
#define all(x) x.begin(), x.end()
#define SZ(x) int(x.size())
const int MXN = 1e5+5;
int n, p[MXN], ans;
vector<string> names;
pair<string, string> edges[MXN];
int GI(string s) { return lower_bound(all(names), s)-names.begin(); }
int vis[MXN];
bool cyc[MXN];
void dfs(int v) {
vis[v] = 1;
if(vis[p[v]]==1) {
for(int u=p[v]; u!=v; u=p[u])
cyc[u] = 1;
cyc[v] = 1;
return;
}
else if(vis[p[v]]==2) return;
dfs(p[v]);
vis[v] = 2;
}
vector<int> g[MXN];
int dp[MXN][2];
void dfs2(int v) {
for(int u : g[v]) {
dfs2(u);
dp[v][1] = max(dp[v][1]+max(dp[u][0], dp[u][1]), dp[v][0]+dp[u][0]+1);
dp[v][0] += max(dp[u][0], dp[u][1]);
}
}
vector<int> vec;
void dfs3(int v) {
vis[v] = 3;
vec.push_back(v);
if(vis[p[v]]!=3) dfs3(p[v]);
}
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
cin >> n;
for(int i=0; i<n; i++) {
cin >> edges[i].X >> edges[i].Y;
names.push_back(edges[i].X);
names.push_back(edges[i].Y);
}
if(n&1) {
cout << "-1\n";
return 0;
}
sort(all(names));
names.resize(unique(all(names))-names.begin());
for(int i=0; i<n; i++)
p[GI(edges[i].X)] = GI(edges[i].Y);
for(int i=0; i<n; i++)
if(p[i]!=i && p[p[i]]==i)
p[p[i]] = p[i],
p[i] = i,
ans += 2;
for(int i=0; i<n; i++)
if(!vis[i])
dfs(i);
for(int i=0; i<n; i++)
if(!cyc[i])
g[p[i]].push_back(i);
for(int i=0; i<n; i++)
if(cyc[i]) {
dfs2(i);
ans += dp[i][1];
}
for(int i=0; i<n; i++)
if(cyc[i] && vis[i]!=3) {
dfs3(i);
bool fnd = 0;
for(int j=0; j<SZ(vec); j++)
if(dp[vec[j]][0]<dp[vec[j]][1]) {
int ln = 0;
for(int k=(j+1)%SZ(vec); k!=j; (++k)%=SZ(vec)) {
if(dp[vec[k]][0]<dp[vec[k]][1]) {
ans += ln/2;
ln = 0;
}
else ln++;
}
ans += ln/2;
fnd = 1;
break;
}
if(!fnd) ans += SZ(vec)/2;
vec.clear();
}
cout << n-ans << '\n';
return 0;
}
