#include<bits/stdc++.h>

#define maxn 1010

#define INF 1010010101

#define eps 1e-9

#define rep1(x,y) for(register int i=x;i<=y;i++)

#define rep2(x,y) for(register int j=x;j<=y;j++)

#define rep3(x,y) for(register int k=x;k<=y;k++)

#define rep4(x,y) for(register int l=x;l<=y;l++)

using namespace std;

typedef long long LL;

typedef unsigned long long ULL;

typedef double db;

bool f[55][55];

LL r,c,n,K,s[55][55];

LL ans,now;

int x,y;

inline int read(){

    int ret=0,f=1;char ch=getchar();

    while (ch<'0'||ch>'9') {if (ch=='-') f=-f;ch=getchar();}

    while (ch>='0'&&ch<='9') ret=ret*10+ch-'0',ch=getchar();

    return ret*f;

}

inline void write(LL x){

    if(x<0){putchar('-'),write(-x);return;}

    if(x/10)write(x/10);putchar(x%10+'0');

}

inline double Read(){

    double X=0,Y=1.0;int w=0;char ch=0;

    while(!isdigit(ch)){w|=ch=='-';ch=getchar();}

    while(isdigit(ch))X=X*10+(ch^48),ch=getchar();

    ch=getchar();

    while(isdigit(ch)) X+=(Y/=10)*(ch^48),ch=getchar();

    return w?-X:X;

}

int main(){

    freopen("Orchestra.in","r",stdin);

    freopen("Orchestra.out","w",stdout);

    r=read();c=read();n=read();K=read();

    rep1(1,n)x=read(),y=read(),f[x][y]=1;

    rep1(1,r)rep2(1,c){s[i][j]=s[i-1][j]+f[i][j]+s[i][j-1]-s[i-1][j-1];}

    for(int i=1;i<=r;i++){

        for(int j=1;j<=c;j++){

            for(int k=i;k<=r;k++){

                for(int l=j;l<=c;l++){

                    now=s[k][l]-s[i-1][l]-s[k][j-1]+s[i-1][j-1];

                    if(now>=K)ans++;

                }

            }

        }

    }

    write(ans);

    return 0;

}