At least n squares must be infected initially. The perimeter of figure(s) formed by infected squares decreases by 0, 2 or 4, depending upon whether the square just infected had 2, 3 or 4 neighbors. When all n2 squares are infected, the perimeter is 4n. So the initial configuration must have at least n infected squares. For example, infecting all diagonals suffices. There are other configurations too (see figure on right).