Finding Sum Of Two Dimensional Array

How to declare a 2 dimensional array of interger of size 10 by 10. and then set every element of the array to 3. After this I need to add up all the element in this array and return the value.

final int ROWS = 10;
final int COLS = 10;
int[][] foo = new int[ROWS][COLS];
for (int i = 0; i < ROWS; i++){
for(int j = 0; i< COLS; i++){
foo[i][j] = 3;
}
}

This is what i have done so far(not 100% if is correct..) but how to add up all the array.