思路见后

#include<bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
long long a[60] = {};
a[1] = 3;
long long cnt = 6;
for(int i = 2; i <= n; i++) {
a[i] = a[i - 1] + cnt;
cnt += 4;
}
cout << a[n];
return 0;
}
/*

| i | a[i]|
| : | : |
| 1 | 3 |
| 2 | 9 |
| 3 | 19 |
| 4 | 33 |

*/