No math, my math is bad:

The idea is each time, we encounter a boundary, count++;

```
public int islandPerimeter(int[][] grid) {
int m = grid.length;
if(m == 0) return 0;
int n = grid[0].length;
int[][] dir = {{0,1},{1,0},{-1,0},{0,-1}};
for(int i = 0; i < m; i++){
for(int j = 0; j < n; j++){
if(grid[i][j] == 1){
return helper(grid, dir, i, j);
}
}
}
return 0;
}
int helper(int[][] grid, int[][] dir, int i, int j){
int m = grid.length, n = grid[0].length;
grid[i][j] = -1;
int count = 0;
for(int[] d: dir){
int x = i + d[0];
int y = j + d[1];
if(x < 0 || y < 0 || x >= m || y >= n || grid[x][y] == 0){
count++;
} else {
if(grid[x][y] == 1){
count += helper(grid, dir, x, y);
}
}
}
return count;
}
```