Codeforces Round #475 (Div. 2) C - Alternating Sum

The summation of proportional series must be discussed in categories! ! ! ! ! ! ! ! ! ! ! ! 

 1 #include<bits/stdc++.h>
 2 #define LL long long
 3 #define fi first
 4 #define se second
 5 #define mk make_pair
 6 #define pii pair<int,int>
 7 #define ull unsigned long long
 8 using namespace std;
 9 
10 const int N=1e6+7;
11 const int M=100+7;
12 const int inf=0x3f3f3f3f;
13 const LL INF=0x3f3f3f3f3f3f3f3f;
14 const int mod=1e9 + 9;
15 
16 LL n, a, b, k;
17 char s[N];
18 
19 LL q_pow(LL a, LL b) {
20     LL ans = 1;
21     while(b) {
22         if(b & 1) ans = ans * a % mod;
23         a = a * a % mod; b >>= 1;
24     }
25     return ans;
26 }
27 
28 LL ivn(LL x) {
29     return q_pow(x, mod - 2);
30 }
31 
32 
33 int main() {
34 
35     scanf("%lld%lld%lld%lld", &n, &a, &b, &k);
36     scanf("%s", s);
37     LL w = q_pow(b, k) * ivn(q_pow(a, k)) % mod;
38     w = (w + mod) % mod;
39 
40     LL ans = 0;
41     LL cnt = (n + 1) / k;
42 
43     for (LL i = 0 ; i <k; i ++ ) {
 44          LL ret1 = q_pow (a, n - i) * q_pow (b, i)% mod;
45          LL ret2 = ( 1 - q_pow (w, cnt) + mod)% mod;
46          LL ret3 = ivn (( 1 - w + mod)% mod);
47          LL ret = (((ret1 * ret2)% mod) * ret3)% mod;
48          if (w == 1 ) ret = (ret1 * cnt)% mod;
49          if (s [i] == ' - ' ) ret = - ret;
50          ans = (ans + ret + mod)% mod;
51     }
52     printf("%lld\n", ans);
53     return 0;
54 }
55 /*
56 */