#include <bits/stdc++.h>
using namespace std;
map<string, int> M;
vector<vector<int>> adj;
int type[100005];
bool vis[100005];
vector<int> V;
int no1 = -1, no2 = -1;
array<int, 2> dfs(int c, int p) {
int d0 = 0, d1 = -1e9;
int sum = 0;
vis[c] = true;
V.push_back(c);
int cnt = 0;
for(int n : adj[c]) {
if(n==p || vis[n]) continue;
if(c==no1&&n==no2) continue;
if(c==no2&&n==no1) continue;
array<int, 2> k = dfs(n, c);
d0 += max(k[0], k[1]);
sum += k[1];
d1 = max(d1, k[0] - k[1]);
cnt++;
}
d1 = sum + d1;
if(cnt==0) return {0, 0};
return {d0, 1 + d1};
}
bool vis2[100005];
array<int, 2> dfs2(int c, int p) {
vis2[c] = true;
for(int n :adj[c]) {
if(n==p) continue;
if(n==c) return {-2, -2};
if(vis2[n]) return {c, n};
array<int, 2> k = dfs2(n, c);
if(k[0]==-2) return {-2, -2};
if(k[0]>=0) return k;
}
return {-1, -1};
}
signed main() {
cin.sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int N;
cin >> N;
int cnt = 0;
int i, j;
adj.resize(N);
for(i=0;i<N;i++) {
string s1, s2;
cin >> s1 >> s2;
int v1 = -1, v2 = -1;
if(M.count(s1)) v1 = M[s1];
else {
M[s1] = v1 = cnt;
cnt++;
}
if(M.count(s2)) v2 = M[s2];
else {
M[s2] = v2 = cnt;
cnt++;
}
adj[v1].push_back(v2);
adj[v2].push_back(v1);
}
if(N%2==1) {
cout << "-1";
return 0;
}
for(i=0;i<N;i++) type[i] = -1;
int ans = 0;
for(i=0;i<N;i++) {
if(!vis2[i]) {
array<int, 2> backedge = dfs2(i, -1);
int x = backedge[0], y = backedge[1];
if(x==-2) {
array<int, 2> k = dfs(i, -1);
ans += max(k[0], k[1]);
continue;
}
assert(x!=-1);
V.clear();
vis[x] = vis[y] = true;
int ma = 0;
int val = 0;
no1 = no2 = -1;
for(int n : adj[y]) {
if(!vis[n]) {
array<int, 2> k = dfs(n, -1);
val += max(k[0], k[1]);
}
}
for(int n : adj[x]) {
if(!vis[n]) {
array<int, 2> k = dfs(n, -1);
val += max(k[0], k[1]);
}
}
val++;
ma = max(ma, val);
no1 = i, no2 = x;
vis[x] = vis[y] = false;
for(int k : V) vis[k] = false;
V.clear();
val = 0;
if(!vis[x]) {
array<int, 2> k = dfs(x, -1);
val += max(k[0], k[1]);
}
if(!vis[y]) {
array<int, 2> k = dfs(y, -1);
val += max(k[0], k[1]);
}
V.clear();
ma = max(ma, val);
ans += ma;
}
}
cout << N - ans;
}
