#include <bits/stdc++.h>
#define F first
#define S second
#define SZ(x) ((int)(x).size())
#define ALL(x) x.begin(), x.end()
#define MP make_pair
using namespace std;
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vii = vector<pii>;
using vll = vector<pll>;
void sol() {
int n;
cin >> n;
vi a(n);
for (int i = 0; i < n; i++) cin >> a[i];
ll ans1 = 0;
for (int i = 2; i < n ;i+=2)
ans1 += a[i] - a[i - 1] + 1;
ll ans2 = 0;
for (int i = 1; i < n ;i+=2)
ans2 += a[i] - a[i - 1] + 1;
cout << min(ans1, ans2) << '\n';
}
signed main() {
cin.tie(0)->sync_with_stdio(0);
sol();
}
