Codeforces #269 (Div. 2)C. MUH and House of Cards（数学：通项公式）

#include <bits/stdc++.h>
#define MAXN 1000010
#define LL long long
using namespace std;

bool vis[MAXN];

int main(void) {

//3*x == n+y;
LL n, x, y, tmp, cnt;
cin >> n;
x = n/3+1;
memset(vis, 0, sizeof(vis));
cnt = 0;
while(true) {
y = 3*x-n;
tmp = y*(y+1)>>1;
if(y > x || tmp>x) {
break;
}
else if(!vis[y]) {
++cnt;
vis[y] = true;
}
++x;
}
cout << cnt << endl;

return 0;
}