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

等比数列求和一定要分类讨论！！！！！！！！！！！！

 #include<bits/stdc++.h>
 #define LL long long
 #define fi first
 #define se second
 #define mk make_pair
 #define pii pair<int,int>
 #define ull unsigned long long
 using namespace std;

 const int N=1e6+;
 const int M=+;
 const int inf=0x3f3f3f3f;
 const LL INF=0x3f3f3f3f3f3f3f3f;
 const int mod=1e9 + ;

 LL n, a, b, k;
 char s[N];

 LL q_pow(LL a, LL b) {
     LL ans = ;
     while(b) {
         if(b & ) ans = ans * a % mod;
         a = a * a % mod; b >>= ;
     }
     return ans;
 }

 LL ivn(LL x) {
     return q_pow(x, mod - );
 }

 int main() {

     scanf("%lld%lld%lld%lld", &n, &a, &b, &k);
     scanf("%s", s);
     LL w = q_pow(b, k) * ivn(q_pow(a, k)) % mod;
     w = (w + mod) % mod;

     LL ans = ;
     LL cnt = (n + ) / k;

     for(LL i = ; i < k; i++) {
         LL ret1 = q_pow(a, n - i) * q_pow(b, i) % mod;
         LL ret2 = ( - q_pow(w, cnt) + mod) % mod;
         LL ret3 = ivn(( - w + mod) % mod);
         LL ret = (((ret1 * ret2) % mod) * ret3) % mod;
         if(w == ) ret = (ret1 * cnt) % mod;
         if(s[i] == '-') ret = -ret;
         ans = (ans + ret + mod) % mod;
     }
     printf("%lld\n", ans);
     return ;
 }
 /*
 */