#include<bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using pi = pair<int,int>;
typedef long long ll;
#define debug(x) cout << #x << " = " << x << "\n";
#define vdebug(a) cout << #a << " = "; for(auto x: a) cout << x << " "; cout << "\n";
const int MOD = 1e9 + 7;
const int INF = 1e9;
template<ll mod>
struct modnum {
static constexpr bool is_big_mod = mod > numeric_limits<int>::max();
using S = conditional_t<is_big_mod, ll, int>;
using L = conditional_t<is_big_mod, __int128, ll>;
S x;
modnum() : x(0) {}
modnum(ll _x) {
_x %= static_cast<ll>(mod);
if (_x < 0) { _x += mod; }
x = _x;
}
modnum pow(ll n) const {
modnum res = 1;
modnum cur = *this;
while (n > 0) {
if (n & 1) res *= cur;
cur *= cur;
n /= 2;
}
return res;
}
modnum inv() const { return (*this).pow(mod-2); }
modnum& operator+=(const modnum& a){
x += a.x;
if (x >= mod) x -= mod;
return *this;
}
modnum& operator-=(const modnum& a){
if (x < a.x) x += mod;
x -= a.x;
return *this;
}
modnum& operator*=(const modnum& a){
x = static_cast<L>(x) * a.x % mod;
return *this;
}
modnum& operator/=(const modnum& a){ return *this *= a.inv(); }
friend modnum operator+(const modnum& a, const modnum& b){ return modnum(a) += b; }
friend modnum operator-(const modnum& a, const modnum& b){ return modnum(a) -= b; }
friend modnum operator*(const modnum& a, const modnum& b){ return modnum(a) *= b; }
friend modnum operator/(const modnum& a, const modnum& b){ return modnum(a) /= b; }
friend bool operator==(const modnum& a, const modnum& b){ return a.x == b.x; }
friend bool operator!=(const modnum& a, const modnum& b){ return a.x != b.x; }
friend bool operator<(const modnum& a, const modnum& b){ return a.x < b.x; }
friend ostream& operator<<(ostream& os, const modnum& a){ os << a.x; return os; }
friend istream& operator>>(istream& is, modnum& a) { ll x; is >> x; a = modnum(x); return is; }
};
using mint = modnum<MOD>;
template <class T> class SumSegmentTree {
private:
const T DEFAULT = 0;
vector<T> segtree;
int len;
public:
SumSegmentTree(int len) : len(len), segtree(len * 2, DEFAULT) {}
void set(int ind, T val) {
ind += len;
segtree[ind] = val;
for (; ind > 1; ind /= 2) {
segtree[ind / 2] = segtree[ind] + segtree[ind ^ 1];
}
}
T range_sum(int start, int end) {
T sum = DEFAULT;
for (start += len, end += len; start < end; start /= 2, end /= 2) {
if (start % 2 == 1) { sum += segtree[start++]; }
if (end % 2 == 1) { sum += segtree[--end]; }
}
return sum;
}
};
struct BIT {
vector<ll>b;
int n;
void init(int _n) {
n = _n ;
b.assign(n+1, 0);
}
inline int lowbit(int x) { return x & (-x); }
void update(int x, ll v) {
++x;
for(int i = x ; i <= n ; i += lowbit(i)) b[i] += v;
}
ll query(int x) {
ll ans = 0;
++x;
for(int i = x ; i > 0 ; i -= lowbit(i)) ans += b[i];
return ans;
}
} bit;
int gcd(int a, int b, int& x, int& y) { // x*a + y*b = a1
x = 1, y = 0;
int x1 = 0, y1 = 1, a1 = a, b1 = b;
while (b1) {
int q = a1 / b1;
tie(x, x1) = make_tuple(x1, x - q * x1);
tie(y, y1) = make_tuple(y1, y - q * y1);
tie(a1, b1) = make_tuple(b1, a1 - q * b1);
}
return a1;
}
int inv_ecd(int a, int m) {
int x, y;
int g = gcd(a, m, x, y);
if(g != 1) return -1;
x = (x + m) % m;
return x;
}
ll exp(ll x, ll n) {
x %= MOD;
ll res = 1;
while (n > 0) {
if (n % 2 == 1) { res = res * x % MOD; }
x = x * x % MOD;
n /= 2;
}
return res;
}
int ans, tmp, e;
vi visited, in, adj;
void dfs(int pos) {
in[pos] = e++;
visited[pos] = 1;
int nxt = adj[pos];
if(visited[nxt] == 1) {
tmp = nxt;
ans += (in[pos] - in[nxt] + 1) % 2;
return;
}
if(visited[nxt] == 2) {
tmp = pos;
return;
}
dfs(nxt);
}
void mark(int pos) {
visited[pos] = 2;
int nxt = adj[pos];
if(visited[nxt] == 2) return;
mark(nxt);
}
void solve() {
int n;
cin >> n;
map<string,int>ind;
int cnt = 1;
adj.resize(n+1);
for(int i=0; i<n; ++i) {
string s, t;
cin >> s >> t;
if(!ind[s]) {
ind[s] = cnt++;
}
if(!ind[t]) {
ind[t] = cnt++;
}
int u = ind[s], v = ind[t];
adj[u] = v;
}
if(n%2) {
cout << -1 << endl;
return;
}
ans = n/2;
e = 0;
int ans2 = 0;
visited.assign(n+1, 0);
in.assign(n+1, 0);
for(int i=1; i<=n; ++i) {
tmp = -1;
if(adj[adj[i]] == i && adj[i] != i) {
visited[i] = visited[adj[i]] = 2;
--ans;
}
if(!visited[i]) {
dfs(i);
if(tmp != i) ans2 += (in[tmp] - in[i] + 1) % 2;
mark(i);
}
}
ans += ans2/2;
cout << ans << endl;
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int tc = 1;
while(tc--) {
solve();
}
}
