A land without neighbor lands would have 4 edges contributing to perimeter. With each neighbor land 1 edge gets cancelled...

```
int islandPerimeter(vector<vector<int>>& grid) { // 132 ms, O(m*n)
int res = 0;
for (int i = 0; i < grid.size(); i++) {
for (int j = 0; j < grid[0].size(); j++) {
if (grid[i][j] == 1) {
int neighbors = 0;
if (i - 1 >= 0)
neighbors += grid[i - 1][j];
if (i + 1 < grid.size())
neighbors += grid[i + 1][j];
if (j - 1 >= 0)
neighbors += grid[i][j - 1];
if (j + 1 < grid[0].size())
neighbors += grid[i][j + 1];
res += 4 - neighbors;
}
}
}
return res;
}
```