, decltype(cmp)> pq(cmp);

vector> level(m, vector(n, INT_MAX));
for (int i = 0; i < m; ++i) {
pq.push({i, 0});
level[i][0] = heights[i][0];
pq.push({i, n-1});
level[i][n-1] = heights[i][n-1];
}
for (int i = 1; i < n-1; ++i) {
pq.push({0, i});
level[0][i] = heights[0][i];
pq.push({m-1, i});
level[m-1][i] = heights[m-1][i];
}
while (!pq.empty()) {
int x = pq.top().first;
int y = pq.top().second;
pq.pop();
if (x>1) {
int l = max(heights[x-1][y], min(level[x-1][y], level[x][y])
);
if (level[x-1][y] != l) {
level[x-1][y] = l;
pq.push({x-1,y});
}
}
if (x < m-1) {
int l = max(heights[x+1][y], min(level[x+1][y], level[x][y])
);
if (level[x+1][y] != l) {
level[x+1][y] = l;
pq.push({x+1, y});
}
}
if (y>1) {
int l = max(heights[x][y-1], min(level[x][y-1], level[x][y])
);
if (level[x][y-1] != l) {
level[x][y-1] = l;
pq.push({x,y-1});
}
}
if (y < n-1) {
int l = max(heights[x][y+1], min(level[x][y+1], level[x][y])
);
if (level[x][y+1] != l) {
level[x][y+1] = l;
pq.push({x, y+1});
}
}
}
int sum = 0;
for (int i = 0; i < m; ++i) {
for (int j = 0; j < n; ++j) {
sum+= level[i][j] - heights[i][j];
}
}
return sum;
}
};