codeforces15E Triangles

传送门：http://codeforces.com/problemset/problem/15/E

【题解】

 # include <bits/stdc++.h>
 using namespace std;

 typedef long long ll;

 const int mod = 1e9 + ;

 int main() {
     int n;
     cin >> n;
     n >>= ;

     int t = , tem, res = , pwr = ;
     for (int i=; i<=n; ++i) {
         pwr <<= ;
         if(pwr >= mod) pwr -= mod;
         tem = pwr - ;
         if(tem <= mod) tem += mod;
         t = 1ll * t * tem % mod;
         res = res + t;
         if(res >= mod) res -= mod;
     }

     res = 4ll * res % mod;

     int ans = 8ll * res % mod + ;
     if(ans >= mod) ans -= mod;
     ans = ans + 2ll * res * res % mod;
     if(ans >= mod) ans -= mod;

     cout << ans;

     return ;
 }