#include <stdio.h>
#include <string.h>
#define N 20
#define INF 0x3f3f3f3f
int min(int a, int b) { return a < b ? a : b; }
int main() {
static int aa[N], dp[1 << N][N][2], dq[1 << N][N][2];
int n, i, b, t, s, s_, ans;
scanf("%d", &n);
for (i = 0; i < n; i++)
scanf("%d", &aa[i]);
for (b = 0; b < 1 << n; b++)
for (i = 0; i < n; i++)
dp[b][i][0] = dp[b][i][1] = INF;
dp[0][0][0] = 0;
for (t = 0; t <= (n - 1) * 2; t++) {
s = 1 << (t + 1) / 2, s_ = 1 << (t + 2) / 2;
for (b = 0; b < 1 << n; b += s)
for (i = 0; i < n; i++)
dq[b][i][0] = dq[b][i][1] = INF;
for (b = 0; b < 1 << n; b += s)
for (i = 0; i < n; i++) {
if ((b & 1 << i) == 0 && aa[i] == 1)
dp[b | 1 << i][i][1] = min(dp[b | 1 << i][i][1], dp[b][i][0]);
if (aa[i] == -1)
dp[b][i][0] = min(dp[b][i][0], dp[b][i][1]);
}
for (b = 0; b < 1 << n; b += s)
for (i = 0; i < n; i++) {
dq[b][i][0] = min(dq[b][i][0], dp[b][i][0]);
dq[b][i][1] = min(dq[b][i][1], dp[b][i][1]);
if (i > 0) {
dq[b][i - 1][0] = min(dq[b][i - 1][0], dp[b][i][0]);
dq[b][i - 1][1] = min(dq[b][i - 1][1], dp[b][i][1]);
}
if (i + 1 < n) {
dq[b][i + 1][0] = min(dq[b][i + 1][0], dp[b][i][0]);
dq[b][i + 1][1] = min(dq[b][i + 1][1], dp[b][i][1]);
}
}
if (t % 2 == 0)
for (b = 0; b < 1 << n; b += s_)
for (i = 0; i < n; i++) {
if (aa[t / 2] == 1)
dq[b][i][0]++, dq[b][i][1]++;
dq[b][i][0] = min(dq[b][i][0], dq[b | 1 << t / 2][i][0]);
dq[b][i][1] = min(dq[b][i][1], dq[b | 1 << t / 2][i][1]);
}
for (b = 0; b < 1 << n; b += s_)
for (i = 0; i < n; i++)
dp[b][i][0] = dq[b][i][0], dp[b][i][1] = dq[b][i][1];
for (b = 0; b < 1 << n; b += s_)
for (i = 0; i < n; i++) {
if ((b & 1 << i) == 0 && aa[i] == 1)
dp[b | 1 << i][i][1] = min(dp[b | 1 << i][i][1], dp[b][i][0]);
if (aa[i] == -1)
dp[b][i][0] = min(dp[b][i][0], dp[b][i][1]);
}
}
ans = INF;
for (i = 0; i < n; i++)
ans = min(ans, min(dp[0][i][0], dp[0][i][1]));
printf("%d\n", ans);
return 0;
}
