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# pairとmapを使いこなす練習　ABC152　D問題

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# Atcorder152 D問題

pairmapを使えば一気に分かりやすくなった！

```#include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(n); i++)
using namespace std;
using ll = long long;
const ll INF = 1e9 + 7;
typedef pair<int,int> P;// pair型は２つの値の組を表せる

P f(int x){
int a=x%10; //下一桁
int b =0;   //上一桁
while(x){
b=x;
x/=10;
}
return P(a,b);
}

int main(){
int n; cin >> n;
map<P,int> freq; //連想配列の時に用いられる
for (int i=1; i<=n; i++){
P p=f(i);
freq[p]++;
}

ll ans=0;
for (int i=1; i<=n; i++){
P p=f(i);
P q(p.second,p.first);
ans+=freq[q]; //A.firstとB.second, A.secondとB.firstが一致しているのか確認
}
cout << ans << endl;
return 0;
}
```
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