Codeforces Round #191 (Div. 2) C. Magic Five

 (a/b)%mod，等价于(a*b^(mod-2))%mod,mod为素数

code:

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<string>
#include<queue>
#include<stack>
#define inf 0x3f3f3f3f3f
using namespace std;
typedef __int64 (ll);
const int maxn = 1e5+5;
const int mod = 1000000007;
char s[maxn];
ll pow(ll a, ll b){
	ll r=1, base=a;
	while(b){
		if(b&1) r= (r*base)%mod;
		base= (base*base)%mod;
		b>>=1;
	}
	return r;
}
__int64 ans=0;
int main(){
   //freopen("in.txt", "r", stdin);
   while(~scanf("%s", s)){
   		ll k;
   		scanf("%I64d", &k);
   		ans=0;
   		ll q=pow(2, strlen(s));
   		for(ll i=0; i<strlen(s); i++){
   			if(s[i]=='0' || s[i]=='5'){
   				ans = (ans+pow(2,i))%mod;	
			}	
		}
		ans = ((ans*(pow(q, k)-1))%mod*pow(q-1,mod-2))%mod;
   		printf("%I64d\n", ans);
   }
   return 0;
}