#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
#define all(v) v.begin(), v.end()
#define pb push_back    
#define sz(x) (int)(x).size()
const int N = 3e5 + 5;
void solve() {
    int n;
    scanf("%d", &n);
    vi a(n);
    for(int &x : a) {
        scanf("%d", &x);
    }
    int last = 0, ans = 0, o = 0, z = 0;
    for(int i = 0; i < n; ++i) { 
        o += (a[i] == 1);
        z += (a[i] == 0);
        if(z && o) {
            ++ans;
            o = z = 0;
        }
    }
    printf("%d", ans);

}
int main() {
    int t = 1;
    // scanf("%d",&t);
    while(t--)
        solve();
}