204B - Little Elephant and Cards - CodeForces Solution

#include <bits/stdc++.h>

using namespace std;

int main() {
ios_base::sync_with_stdio(0);
unordered_set<int> st;
unordered_map<int, int> aa, bb;
int n;
cin >> n;
for (int i = 0; i < n; i++) {
int a, b;
cin >> a >> b;
aa[a]++;
if (a != b) bb[b]++;
st.insert(a);
st.insert(b);
}
int ans = 1e9;
for (auto i: st) {
if (aa[i] >= n/2+n%2) {
ans = 0;
} else if (aa[i] + bb[i] >= n/2+n%2) {
ans = min(ans, n/2+n%2 - aa[i]);
}
}
if (ans == 1e9) cout << "-1\n";
else cout << ans << '\n';
}