#include <bits/stdc++.h>
using namespace std;
#define int int64_t
signed main() {
int n;cin>>n;
if (n % 2 != 0) {
cout<<-1<<"\n";
return 0;
}
map<string,int> mp;
vector<vector<int>> in(n);
vector<int> out(n);
int cnt=0;
for (int i=0;i<n;i++) {
string s1, s2;cin>>s1>>s2;
if (mp.find(s1) == mp.end()) {
mp[s1] = cnt++;
}
if (mp.find(s2) == mp.end()) {
mp[s2] = cnt++;
}
if (mp[s1] != mp[s2])
in[mp[s2]].push_back(mp[s1]);
out[mp[s1]] = mp[s2];
}
// remove all relationships (cycles of length 2)
int swaps = n;
vector<bool> vis1(n, false);
for (int i=0;i<n;i++) {
if (vis1[i])continue;
if (out[out[i]] != i || out[i] == i) continue;
vis1[i] = true;
vis1[out[i]] = true;
for (auto &x:in[i]) {
out[x] = x;
}
for (auto &x:in[out[i]]) {
out[x] = x;
}
swaps -= 2;
}
// get rid of all cycles of length 1 (people that love themselves), and the respective lines connected to them
vector<int> dpWith(n, 0), dpWithout(n, 0);
function<void(int)> dfs1=[&](int node) {
vis1[node] = true;
int sm = 0;
for (auto &x:in[node]) {
dfs1(x);
dpWithout[node] += max(dpWith[x], dpWithout[x]);
sm += max(dpWith[x], dpWithout[x]);
}
for (auto &x:in[node]) {
dpWith[node] = max(dpWith[node], sm - max(dpWith[x], dpWithout[x]) + dpWithout[x] + 1);
}
};
for (int i=0;i<n;i++) {
if (out[i] == i && !vis1[i]) {
dfs1(i);
swaps -= max(dpWith[i], dpWithout[i]);
}
}
// find all nodes that are part of a cycle
vector<bool> incycle(n, false);
vector<int> stat(n, 0);
vector<bool> vis = vis1;
function<int(int)> dfs2=[&](int node) -> int {
vis[node] = true;
// still being processed
stat[node] = 1;
if (stat[out[node]] == 0) {
int d = dfs2(out[node]);
if (d != -1) {
incycle[node] = true;
}
if (d == node) {
d = -1;
}
stat[node] = 2;
return d;
}
else if (stat[out[node]] == 1) {
stat[node] = 2;
incycle[node] = true;
return out[node];
}
// finished processing
stat[node] = 2;
return -1;
};
for (int i=0;i<n;i++) {
if (!vis[i]) dfs2(i);
}
// calculate dp values for all nodes in a cycle
function<void(int)> dfs3=[&](int node) {
int sm = 0;
for (auto &x:in[node]) {
if (incycle[x])continue;
dfs3(x);
dpWithout[node] += max(dpWith[x], dpWithout[x]);
sm += max(dpWith[x], dpWithout[x]);
}
for (auto &x:in[node]) {
if (incycle[x])continue;
dpWith[node] = max(dpWith[node], sm - max(dpWith[x], dpWithout[x]) + dpWithout[x] + 1);
}
};
for (int i=0;i<n;i++) {
if (incycle[i]) {
dfs3(i);
swaps -= dpWith[i];
}
}
vis.clear();
vis.assign(n, false);
for (int i=0;i<n;i++) {
if (incycle[i] && !vis[i]) {
int pt = i;
vector<bool> pos;
do {
pos.push_back(dpWith[pt] == dpWithout[pt]);
vis[pt] = true;
pt = out[pt];
} while (pt != i);
vector<bool> tmp;
while (pos.size() && pos.back() == true) {
tmp.push_back(true);
pos.pop_back();
}
for (bool x:pos) {
tmp.push_back(x);
}
tmp.push_back(false);
stack<bool> st;
for (bool x:tmp) {
if (x) {
st.push(x);
}
else {
swaps -= ((int)st.size() / 2);
while (st.size()) st.pop();
}
}
}
}
cout<<swaps<<"\n";
}
