题解 | #等差数列#

#include <bits/stdc++.h>
using namespace std;
const int N = 1010;
int s[N];
void getPrefixSum(vector<int>& nums){
    int n = nums.size();
    s[0] = nums[0];
    for(int i = 1; i <= n; ++i){
        s[i] = s[i - 1] + nums[i];
    }

}
int main() {
    int n;
    cin >> n;
    vector<int> f(n);
    f[0] = 2;
    for(int i = 1; i < n; ++i){
        f[i] = f[i - 1] + 3;
    }
    getPrefixSum(f);
    cout << s[n - 1] << endl;

    return 0;
}
// 64 位输出请用 printf("%lld")