CodeForces 37d

#include<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
const int N=1100;
const int mod=1e9+7;
const int INF=0x3f3f3f3f;
typedef __int64 LL;

LL jie
,inv
;
LL _pow(LL a,int b){
LL c=1;
while(b){
if(b&1){
c*=a;c%=mod;
}
a*=a;a%=mod;b/=2;
}
return c;
}

void init(){
jie[0]=inv[0]=1;
int n=1000;
for(int i=1;i<=n;i++){
jie[i]=jie[i-1]*i%mod;
}
for(int i=n;i>=1;i--){
if(i==n)inv[i]=_pow(jie[i],mod-2);
else {
inv[i]=inv[i+1]*(i+1)%mod;
}
}
}

LL C(int i,int j){
if(i<j)return 0;
return jie[i]*inv[j]%mod*inv[i-j]%mod;
}

int xx
,yy
;
int sum,s
;
LL dp

;
int vis

;

int main(){
#ifdef DouBi
freopen("in.cpp","r",stdin);
#endif // DouBi
int m;
init();
//    printf("%I64d\n",C(3,1));
while(scanf("%d",&m)!=EOF){
sum=0;
for(int i=0;i<m;i++){
scanf("%d",&xx[i]);
sum+=xx[i];
s[i]=xx[i];
if(i)s[i]+=s[i-1];
}
for(int i=0;i<m;i++){
scanf("%d",&yy[i]);
}

memset(dp,0,sizeof(dp));
memset(vis,0,sizeof(vis));
for(int i=0;i<m;i++){
for(int j=0;j<=s[i];j++){
for(int k=0;k<=yy[i]&&k<=j;k++)if(i==0&&j-k==0||i&&vis[i-1][j-k]){
vis[i][j]=1;
LL x=C(s[i]-(j-k),k);
if(i)x*=dp[i-1][j-k];
x%=mod;
dp[i][j]+=x;
dp[i][j]%=mod;
}
}
}
LL ans=dp[m-1][sum];

for(int i=0;i<m;i++){
if(i)ans*=C(sum-s[i-1],xx[i]);
else ans*=C(sum,xx[i]);
ans%=mod;
}
printf("%I64d\n",ans);
}
return 0;
}