Codeforces Round #419 (Div. 1)B - Karen and Test

 #include<bits/stdc++.h>

using namespace std;

#define X first
#define Y second
#define eps  1e-2
#define gcd __gcd
#define pb push_back
#define PI acos(-1.0)
#define lowbit(x) (x)&(-x)
#define bug printf("!!!!!\n");
#define mem(x,y) memset(x,y,sizeof(x))
#define int long long


typedef long long LL;
typedef long double LD;
typedef pair<int,int> pii;
typedef unsigned long long uLL;

const int maxn = 2e5+2;
const int INF  = 1<<30;
const int mod  = 1e9+7;

int fac[maxn]={1,1},inv[maxn]={1,1},fi[maxn]={1,1};
int a[maxn],n;
void init(){
    for(int i=2;i<=maxn/2;i++){
        fac[i]=fac[i-1]*i%mod;
        inv[i]=(mod-mod/i)*inv[mod%i]%mod;
        fi[i]=fi[i-1]*inv[i]%mod;
    }
}
 
LL c(LL n, LL m){
    return fac[n]*fi[n-m]%mod*fi[m]%mod;
}

void solve(){
    init();
    scanf("%lld",&n);
    for(int i=1;i<=n;i++) scanf("%lld",&a[i]);
    if(n<=2){
        cout<<(a[1]+a[2])%mod<<endl;
        return ;
    }
    if(n%2){
        int f=1;
        for(int i=1;i<n;i++){
            a[i]=a[i]+f*a[i+1];
            f=-f;
        }
        --n;
    }
    int res=0,f=(n%4!=0?1:-1);
    for(int i=1;i<=n;i+=2){
        int base=c(n/2-1,i/2);
        // cout<<base<<endl;
        res=(res+(a[i]+a[i+1]*f)*base+mod)%mod;
    }
    cout<<(res+mod)%mod<<endl;
    return;
}

int32_t main()
{
//    freopen("in.txt","r",stdin);
//    freopen("out.txt","w",stdout);
//    ios::sync_with_stdio(false);
    int t = 1;
    //scanf("%d",&t);
    while(t--){
    //    printf("Case %d: ",cas++);
        solve();
    }
    return 0;
}