#include <bits/stdc++.h>
using namespace std;
int n;
int nxt[100000], f[2][100000], h[2][2][100000];
bool check[100000];
string p[100000];
map<string, int> mp;
vector<int> g[100000], temp;
vector<vector<int>> v;
inline void Subtask1()
{
vector<int> g;
g.resize(1 << n, 1e9);
g[0] = 0;
for (int i = 0; i < (1 << n); ++i)
{
if (__builtin_popcount(i) & 1)
{
continue;
}
for (int j = 0; j < n; ++j)
{
for (int k = j + 1; k < n; ++k)
{
if (((i >> j) & 1) && ((i >> k) & 1))
{
g[i] = min(g[i], (nxt[j] != k) + (nxt[k] != j) + g[i - (1 << j) - (1 << k)]);
}
}
}
}
cout << g[(1 << n) - 1];
}
inline void DFS(int node)
{
vector<int> cur;
check[node] = true;
temp.push_back(node);
if (check[nxt[node]])
{
while (!temp.empty())
{
cur.push_back(temp.back());
if (temp.back() == nxt[node])
{
break;
}
temp.pop_back();
}
if (cur.back() == nxt[node])
{
reverse(cur.begin(), cur.end());
v.push_back(cur);
}
}
else
{
DFS(nxt[node]);
}
}
inline void DFS1(int node)
{
f[0][node] = 1;
f[1][node] = 1e9;
for (auto &i : g[node])
{
if (!check[i])
{
DFS1(i);
f[1][node] = min(f[1][node] + min(f[1][i], f[0][i]), f[0][node] + f[0][i] - 1);
f[0][node] += min(f[0][i], f[1][i]);
}
}
}
inline void Subtask4()
{
int res = 0;
for (int i = 0; i < n; ++i)
{
if (!check[i])
{
temp.clear();
DFS(i);
}
}
fill_n(check, n, 0);
for (auto &i : v)
{
for (auto &j : i)
{
check[j] = true;
}
}
for (auto &i : v)
{
for (auto &j : i)
{
DFS1(j);
}
if (i.size() == 1)
{
res += min(f[1][i.back()], f[0][i.back()]);
continue;
}
for (int j = 0; j < 2; ++j)
{
for (int k = 0; k < 2; ++k)
{
fill_n(h[j][k], i.size(), 1e9);
}
}
// h[i][j][k] : i for the first one take / pass, j for the k'th one take / pass, k is the index
h[0][0][1] = f[0][i[0]] + f[0][i[1]];
h[0][1][1] = f[0][i[0]] + f[1][i[1]];
h[1][0][1] = f[1][i[0]] + f[0][i[1]];
h[1][1][1] = min(f[1][i[0]] + f[1][i[1]], f[0][i[0]] + f[0][i[1]] - 1);
for (int j = 2; j < i.size(); ++j)
{
h[0][0][j] = f[0][i[j]] + min(h[0][0][j - 1], h[0][1][j - 1]);
h[0][1][j] = min(f[1][i[j]] + min(h[0][0][j - 1], h[0][1][j - 1]), // count independently
f[0][i[j]] + h[0][0][j - 1] - 1); // connect with the previous one
h[1][0][j] = f[0][i[j]] + min(h[1][0][j - 1], h[1][1][j - 1]);
h[1][1][j] = min(f[1][i[j]] + min(h[1][0][j - 1], h[1][1][j - 1]),
f[0][i[j]] + h[1][0][j - 1] - 1);
}
res += min({h[0][0][i.size() - 1] - 1 - (i.size() == 2), h[0][1][i.size() - 1], h[1][0][i.size() - 1], h[1][1][i.size() - 1]});
}
cout << res;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
for (int i = 0; i < n; ++i)
{
cin >> p[i];
mp[p[i]] = i;
cin >> p[i];
}
for (int i = 0; i < n; ++i)
{
nxt[i] = mp[p[i]];
g[mp[p[i]]].push_back(i);
}
if (n & 1)
{
cout << -1;
return 0;
}
if (n <= 20)
{
Subtask1();
return 0;
}
Subtask4();
return 0;
}
