#include <bits/stdc++.h>
#define N 100010

using namespace std;

int n, tot, vis[N], sz[N], num[N], del[N], to[N], in[N], flag, ans;
queue <int> q;
unordered_map <string, int> book;

inline int New(string x) {
    if(book[x])
        return book[x];
    return book[x] = ++tot;
}

inline void topu() {
    for(int i = 1; i <= n; i++)
        if(!in[i] && !vis[i])
            q.push(i);
    
    while(!q.empty()) {
        int u = q.front();
        q.pop();
        ++ans;

        int v = to[u];
        if(!vis[v]) {
            vis[v] = 1;
            --in[to[v]];
            if(!in[to[v]] && !vis[to[v]])
                q.push(to[v]);
        }
    }
}

int dfs(int u) {
    if(vis[u])
        return 0;
    vis[u] = 1;
    return dfs(to[u]) + 1;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin >> n;

    for(int i = 1; i <= n; i++) {
        string x, y;
        cin >> x >> y;
        int u = New(x), v = New(y);
        if(u == v)
            continue;
        to[u] = v;
    }

    if(n & 1) {
        cout << -1 << endl;
        return 0;
    }
    
    vis[0] = 1;
    for(int i = 1; i <= n; i++) {
        if(i == to[to[i]] && !vis[i] && !vis[to[i]])
            vis[i] = vis[to[i]] = 1;
    }

    for(int i = 1; i <= n; i++) {
        if(!vis[i])
            ++in[to[i]];
    }
    topu();

    for(int i = 1; i <= n; i++) {
        int res = dfs(i);
        if(res <= 1)
            continue;
        ans += res & 1;
        ans += res >> 1;
    }
    cout << ans << endl;
}