#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#include<cstdlib>
using namespace std;
int n,m,k,s,dp[110][110],a[110],b[110],minn;
int main()
{
while(scanf("%d%d%d%d",&n,&m,&k,&s)!=EOF)
{
 memset(a,0,sizeof(a));
 memset(b,0,sizeof(b));
 memset(dp,0,sizeof(dp));
 for(int i=1;i<=k;i++)
     scanf("%d%d",&a[i],&b[i]);
 for(int i=1;i<=k;i++)
 	 for(int j=1;j<=s;j++)
 	     for(int p=b[i];p<=m;p++)
 	         dp[j][p]=max(dp[j][p],dp[j-1][p-b[i]]+a[i]);
 if(dp[s][m]<n)
    printf("-1\n");
 else
 {
 	minn=m;
 	for(int i=0;i<=s;i++)
 	for(int j=0;j<=m;j++)
 	       if(dp[i][j]>=n&&j<minn)
 	          minn=j;
 	   printf("%d\n",m-minn);
 }
 
}
return 0;
}