题解 | #等差数列#

let line;

function func(line) {
    let rtnVal;
    const n = parseInt(line);
    const a1 = 2;
    const d = 3;
    
    // 前n项和公式为：Sn=n*a1+n(n-1)d/2或Sn=n(a1+an)/2
    rtnVal = n * (a1 + d * (n - 1) / 2);
    return rtnVal;
}

while(line = readline()){
   print(func(line));
}