//
// Created by mihai145 on 26.02.2021.
//
#include <iostream>
#include <vector>
#include <unordered_map>
#include <stack>
#include <algorithm>
using namespace std;
const int NMAX = 1e5;
int N, k;
unordered_map<string, int> names;
vector<int> g[NMAX + 5];
vector<int> bi[NMAX + 5];
bool solvedFor[NMAX + 5];
int Get(string s) {
if (!names[s]) {
names[s] = ++k;
}
return names[s];
}
void dfsConex(int node) {
solvedFor[node] = true;
for (int son : bi[node]) {
if (!solvedFor[son]) {
dfsConex(son);
}
}
}
vector<int> cycle;
bool inStack[NMAX + 5], cycleNode[NMAX + 5];
stack<int> st;
void dfsFindCycle(int node) {
inStack[node] = true;
st.push(node);
for (int son : g[node]) {
if (inStack[son] == true) {
while (!st.empty() && st.top() != son) {
cycle.push_back(st.top());
cycleNode[st.top()] = true;
st.pop();
}
cycle.push_back(st.top());
cycleNode[st.top()] = true;
return;
} else {
dfsFindCycle(son);
}
}
}
int dpTree[NMAX + 5][2], dpCycle[NMAX + 2][2];
void CalcDpTree(int node, int par = -1) {
int nrSons = 0;
for (int son : bi[node]) {
if (son != par && !cycleNode[son]) {
nrSons++;
CalcDpTree(son, node);
}
}
if (nrSons == 0) {
dpTree[node][0] = dpTree[node][1] = 0;
} else {
int maxMatch = -N - 1;
for (int son : bi[node]) {
if (son != par && !cycleNode[son]) {
dpTree[node][0] += max(dpTree[son][0], dpTree[son][1]);
dpTree[node][1] += max(dpTree[son][0], dpTree[son][1]);
maxMatch = max(maxMatch, -max(dpTree[son][0], dpTree[son][1]) + dpTree[son][0]);
}
}
dpTree[node][1] += maxMatch;
dpTree[node][1] += 1;
}
}
int keeps;
void solve(int node) {
dfsConex(node);
while (!st.empty()) {
st.pop();
}
cycle.clear();
dfsFindCycle(node);
// cout << "New cycle:\n";
// for(auto it : cycle) {
// cout << it << ' ';
// }
//
// cout << '\n' << '\n';
for (int vert : cycle) {
CalcDpTree(vert);
}
if ((int) cycle.size() == 1) {
keeps += max(dpTree[cycle[0]][0], dpTree[cycle[0]][1]);
return;
}
reverse(cycle.begin(), cycle.end());
int maxKeeps = 0;
///we do not match the first node in the cycle with the last one
dpCycle[cycle[0]][0] = dpTree[cycle[0]][0];
dpCycle[cycle[0]][1] = dpTree[cycle[0]][1];
for (int i = 1; i < (int) cycle.size(); i++) {
int maxPrev = max(dpCycle[cycle[i - 1]][0], dpCycle[cycle[i - 1]][1]);
dpCycle[cycle[i]][0] = maxPrev + dpTree[cycle[i]][0];
dpCycle[cycle[i]][1] = max(maxPrev + dpTree[cycle[i]][1], dpCycle[cycle[i - 1]][0] + 1 + dpTree[cycle[i]][0]);
}
maxKeeps = max(dpCycle[cycle.back()][0], dpCycle[cycle.back()][1]);
///we match the first node in the cycle with the last one
dpCycle[cycle[0]][0] = dpTree[cycle[0]][0];
dpCycle[cycle[0]][1] = 0;
for (int i = 1; i < (int) cycle.size(); i++) {
int maxPrev = max(dpCycle[cycle[i - 1]][0], dpCycle[cycle[i - 1]][1]);
dpCycle[cycle[i]][0] = maxPrev + dpTree[cycle[i]][0];
if (i > 1) {
dpCycle[cycle[i]][1] = max(maxPrev + dpTree[cycle[i]][1],
dpCycle[cycle[i - 1]][0] + 1 + dpTree[cycle[i]][0]);
} else {
dpCycle[cycle[i]][1] = maxPrev + dpTree[cycle[i]][1];
}
}
maxKeeps = max(maxKeeps, dpCycle[cycle.back()][1]);
maxKeeps = max(maxKeeps, dpCycle[cycle.back()][0] + 1);
if ((int) cycle.size() == 2) {
maxKeeps = max(maxKeeps, 2 + dpTree[cycle[0]][0] + dpTree[cycle[1]][0]);
}
keeps += maxKeeps;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> N;
if (N % 2 == 1) {
cout << -1 << '\n';
return 0;
}
for (int i = 1; i <= N; i++) {
string s, t;
cin >> s >> t;
g[Get(s)].push_back(Get(t));
bi[Get(s)].push_back(Get(t));
bi[Get(t)].push_back(Get(s));
}
for (int i = 1; i <= N; i++) {
if (!solvedFor[i]) {
solve(i);
}
}
cout << N - keeps << '\n';
return 0;
}
