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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3. #define ll long long 
  4. #define mod 1000000007
  5. ll po(ll a,ll n){
  6.         ll res=a,ans=1;
  7.         while(n){
  8.                 if(n%2) ans=ans*res%mod;
  9.                 res=res*res%mod;
  10.                 n/=2;
  11.         }
  12.         return ans;
  13. }
  14. int main(){
  15.         ll mod_4 = po(4,mod-2);
  16.         ll n;
  17.         cin>>n;
  18.         ll k;
  19.         if(n%2==0) {k =n/2;
  20.                 ll res = 9*mod_4;
  21.                 ll tmp = (po(9,k-1)-1+mod)%mod;
  22.                 res = res*tmp%mod;
  23.                 res=(res+3)%mod;
  24.                 cout<<res;
  25.                 return 0;
  26.         }
  27.         else {
  28.                 k=(n-1)/2;
  29.                 ll res = mod_4*3;
  30.                 ll tmp = (po(9,k)-1+mod)%mod;
  31.                 res = res*tmp%mod;
  32.                 cout<<res;
  33.                 return 0;
  34.         }
  35. }
Success #stdin #stdout 0s 4948KB
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https://ideone.com/9DzhkO
language:
C++ (gcc 8.3)
created:
3 years ago
visibility:
public

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