import std.stdio;
import std.typecons;

void main()
{
	int r, c, n, k;
	
	scanf("%d%d%d%d", &r, &c, &n, &k);
	
	Tuple!(int, int)[] alts = new Tuple!(int, int)[n];
	
	for (int i = 0; i < n; i++)
		scanf("%d%d", &alts[i][0], &alts[i][1]);
	
	int res = 0;
	for (int i = 1; i <= r; i++)
		for (int j = 1; j <= c; j++)
			for (int p = i; p <= r; p++)
				for (int q = j; q <= c; q++) {
					int count = 0;
					foreach (Tuple!(int, int) alt; alts)
						if (alt[0] >= i && alt[0] <= p && alt[1] >= j && alt[1] <= q)
							count++;
					if (count >= k)
						res++;
				}
	printf("%d\n", res);
}
