题解 | #等差数列#

#include <stdio.h>
#include <math.h>

int main(){

    int n;
    const int A1 = 2;
    const int D = 3;
    scanf("%d", &n);

    int sn;
    sn = ((float)n / 2) * (2 * A1 + (n - 1) * D);

    printf("%d", sn);
    return 0;
}