#include <algorithm>
#include <cstdio>
#include <ctime>

template <typename T> T read();
template <typename T> void write(T, char);

const int MAXN = 1e5 + 10;
int n;
int a[MAXN];
int ans;

bool check() {
  return ::std::all_of(a + 1, a + n + 1, [](int x) { return x == 0; });
}

void dfs(int now) {
  if (clock() > 1900) {
    return;
  }
  bool ok = false;
  if (now < n && a[now]) {
    ok = true;
    a[now]--;
    dfs(now + 1);
    a[now]++;
  }
  if (now > 0 && a[now - 1]) {
    ok = true;
    a[now - 1]--;
    dfs(now - 1);
    a[now - 1]++;
  }
  if (!ok) (ans += check()) %= int(1e9 + 7);
}

int main() {
  freopen("moorouteg.in", "r", stdin);
  freopen("moorouteg.out", "w", stdout);
  n = read<int>();
  for (int i = 0; i < n; ++i) {
    a[i] = read<int>();
  }

  dfs(0);

  write(ans / 2, '\n');
  return 0;
}

#define isdigit(ch) (ch >= '0' && ch <= '9')
template <typename T> T read() {
  T res = 0, f = 1;
  char ch = getchar();
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') f = -1;
  for (; isdigit(ch); ch = getchar()) res = (res << 3) + (res << 1) + (ch ^ 48);
  return res * f;
}
#undef isdigit

template <typename T> void write(T x, char ed) {
  if (x < 0) x = -x, putchar('-');
  int sta[sizeof(T) << 2], top = 0;
  do {
    sta[++top] = x % 10;
    x /= 10;
  } while (x);
  while (top) {
    putchar(sta[top--] ^ 48);
  }
  putchar(ed);
}