#include <bits/stdc++.h>
using namespace std;
const int maxn = 300;
int dp[maxn][maxn][maxn][2];
int main() {
int n;
cin>>n;
vector<int> vi(n);
for(auto &a:vi)
cin>>a;
for(int i = 0; i< maxn;i++)
for(int j = 0; j < maxn; j++)
for(int k = 0; k < maxn;k++)
for(int l = 0; l < 2; l++)
dp[i][j][k][l] = -1;
auto repmax = [&](int &cur,int x) {
cur = max(cur, x);
};
//tim, pos, last, has
if(vi[0] >= 1) {
dp[0][0][1][1] = 1;
}else
dp[0][0][0][0] = 0;
for(int i = 0; i < 3*n ;i++)
for(int j = 0; j < n; j++)
for(int k = 0;k <= n; k++) {
// cout<<i<<' '<<j<<' '<<k<<' '<<dp[i][j][k][0]<<' '<<dp[i][j][k][1]<<'\n';
if(dp[i][j][k][0]!=-1 ){
//send forward
if(j + 1 < n) {
repmax(dp[i + 1][j+1][k][0], dp[i][j][k][0]);
if(vi[j+1] >= 1 && k <= j + 1 && i + 1 <= 2*(j+1)) {
repmax(dp[i+1][j+1][j + 2][1], dp[i][j][k][0] + 1);
}
}
//send back
if(j - 1>= 0) {
repmax(dp[i + 1][j-1][k][0], dp[i][j][k][0]);
if(vi[j-1] >= 1 && k <= j - 1 && i + 1 <= 2*(j-1)) {
repmax(dp[i+1][j - 1][j][1], dp[i][j][k][0] + 1);
}
}
}
if(dp[i][j][k][1] != -1) {
if(j + 1 < n) {
if(vi[j+1] == -1) {
repmax(dp[i+1][j + 1][k][0], dp[i][j][k][1]);
}else
repmax(dp[i+ 1][j + 1][k][1], dp[i][j][k][1]);
}
if(j - 1 < n) {
if(vi[j - 1] == -1) {
repmax(dp[i + 1][j - 1][k][0], dp[i][j][k][1]);
}else
repmax(dp[i+1][j-1][k][1], dp[i + 1][j - 1][k][1]);
}
}
}
int maxval = 0;
for(int i = 0; i <= 2*n; i++)
for(int j = 0; j < n; j++)
for(int k = 0; k < n; k++)
maxval = max({maxval, dp[i][j][k][0], dp[i][j][k][1]});
cout<<count(vi.begin(), vi.end(), 1) - maxval<<'\n';
}
