#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5e5 + 5;
template <typename T>
void chkmax(T& x, T y) {
  if (y > x) x = y;
}
template <typename T>
void chkmin(T& x, T y) {
  if (y < x) x = y;
}
int N, c[MAXN];
long long ans;
int main() {
  scanf("%d", &N);
  for (int i = 1; i <= N; ++i) scanf("%d", &c[i]);
  for (int i = 1; i <= N; i += 2) {
    long long minv = 0, sum = 0;
    for (int j = i + 1; j <= N; ++j) {
      long long l = -sum - c[i], r = -sum - max(1ll, -minv);
      if (j % 2 == 1)
        chkmax(l, 1ll), chkmin(r, (long long)c[j]);
      else
        chkmax(l, (long long)-c[j]), chkmin(r, -1ll);
      ans += max(0ll, r - l + 1);
      sum += j % 2 == 1 ? c[j] : -c[j];
      chkmin(minv, sum);
    }
  }
  printf("%lld", ans);
  return 0;
}
