俺滚动数组咋不对捏

#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main()
{
    int n;
    cin >> n;
    if (n == 1)cout << 1;
    if (n == 2)cout << 2;
    if (n == 3)cout << 4;
    int a = 1, b = 2, c = 4, d = 0;
    for (int i = 4; i <= n; i++)d = a + b + c, a = b, b = c, c = d;
    cout << d;
    return 0;
}