持续更新——set，map

#include 
#define endl '\n'
#define x first
#define y second
#define int long long
#define Tang ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);

using namespace std;

const int N=1e5+10;
const int INF=0x3f3f3f3f;
const int mod=1e9+7;

map <string,int> q;
string a,b;
int ans=0;

void solve()
{
    
    int n;
    cin >> n;
    while(n--)
    {
        
        cin >> a >> b;
        a=a.substr(0,2);    
        if(a!=b)
            ans+=q[a+b];
        q[b+a]++;
        
    }
    cout << ans << endl;
}

signed main()
{
    Tang
    int T=1;
        //cin >> T;
    while(T--)
        solve();
    
    return 0;
        
}

#include 
#define endl '\n'
#define x first
#define y second
#define int long long
#define Tang ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);

using namespace std;

const int N=1e5+10;
const int INF=0x3f3f3f3f;
const int mod=1e9+7;

map <int,map<int,int>> q;
string a,b;
int ans=0;

void solve()
{
    
    int n;
    cin >> n;
    while(n--)
    {
        
        cin >> a >> b;
        int x=a[0]*100+a[1];
        int y=b[0]*100+b[1];
        if(x!=y)
        {
            ans+=q[x][y];
            q[y][x]++;
        }
        
    }
    cout << ans << endl;
}

signed main()
{
    Tang
    int T=1;
        //cin >> T;
    while(T--)
        solve();
    
    return 0;
        
}