#include <bits/stdc++.h>
#define file_io freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define fast_io ios::sync_with_stdio(false);cin.tie(0);
#define what(x) cerr << #x << " is " << x << '\n';
#define kill(x) {cout << x << '\n'; return 0;}
#define all(x) (x).begin(), (x).end()
#define pii pair<int, int>
#define pb push_back
#define ll long long
#define F first
#define S second
const ll inf = 1e18, mod = 1e9 + 7, delta = 1e9 + 9, SQ = 450, P = 6065621;
using namespace std;
const ll N = 2e5 + 10, LG = 31;
int go[N], c;
unordered_map<string, int> com;
bool mark[N];
void dfs (int v) {
c++;
mark[v] = true;
if (!mark[go[v]])
dfs(go[v]);
}
int main() {
fast_io;
int m, n = 0;
cin >> m;
while (m--) {
string s, t;
cin >> s >> t;
if (!com[s]) com[s] = ++n;
if (!com[t]) com[t] = ++n;
go[com[s]] = com[t];
}
if (n & 1) kill(-1);
vector<int> cnt;
for (int i = 1; i <= n; i++)
if (!mark[i])
c = 0, dfs(i), cnt.pb(c);
int ans = 0;
int cntt = 0;
for (auto u: cnt) {
if (u & 1) cntt++;
ans += u / 2;
}
ans += cntt;
cout << ans << '\n';
return 0;
}
