{

int n = static_cast<int>(grid.size());

if (n == 1) return grid[0][0] == 1 ? -1 : 0;

vector<int> trailing0(n, 0);

for (int i = 0; i < n; ++i) {

int cnt0 = 0;

for (int j = n - 1; j >= 0; --j) {

if (grid[i][j] == 0) cnt0++;

else break;

}

trailing0[i] = cnt0;

}

int steps= 0;

for (int i = 0; i < n; ++i) {

int needed = n - i - 1;

int j = i;

// find first statisfied trailing[j]

while (j < n && trailing0[j] < needed) j++;

if (j == n) return -1;

while (j > i) {

std::swap(trailing0[j], trailing0[j - 1]);

j--;

steps++;

}

}

return steps;