1 #include<cstdio>
 2 #include<cstring>
 3 #include<cstdlib>
 4 #include<cmath>
 5 #include<algorithm>
 6 #define qread(x) x=read()
 7 using namespace std;
 8 inline int read()
 9 {
10     int f=1,x=0;char ch;
11     while(ch<‘0‘ || ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
12     while(ch>=‘0‘ && ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
13     return f*x;
14 }
15 int a[11000];
16 int f[11000],n;
17 int solve()
18 {
19     for(int i=2;i<n;i++)
20     {
21         int t=a[i]-f[i]-f[i-1];
22         if(t<0 || t>1)return 0;
23         else f[i+1]=t;
24     }
25     if(f[n]+f[n-1]!=a[n])return 0;
26     return 1;
27 }
28 int main()
29 {
30     int ans=0;
31     memset(f,0,sizeof(f));
32     qread(n);
33     for(int i=1;i<=n;i++)qread(a[i]);
34     if(a[1]>2){printf("0\n");return 0;}
35     if(a[n]>2){printf("0\n");return 0;}
36     if(a[1]==2)
37     {
38         f[1]=1;f[2]=1;
39         ans+=solve();
40     }
41     if(a[1]==1)
42     {
43         memset(f,0,sizeof(f));
44         f[1]=1;
45         ans+=solve();
46         memset(f,0,sizeof(f));
47         f[2]=1;
48         ans+=solve();
49     }
50     if(a[1]==0)
51         memset(f,0,sizeof(f)),ans+=solve();
52     printf("%d\n",ans);
53     return 0;
54 }