#include<bits/stdc++.h> using namespace std; int f[200][200],n,m,k,s,a[105],b[105]; int main(){ while(~scanf("%d%d%d%d",&n,&m,&k,&s)){ memset(f,0,sizeof(f)); for(int i=1;i<=k;i++)scanf("%d%d",&a[i],&b[i]); for(int i=1;i<=k;i++)for(int v=1;v<=s;v++)for(int u=b[i];u<=m;u++)f[v][u]=max(f[v][u],f[v-1][u-b[i]]+a[i]); int flag=-1; for(int u=0;u<=m;u++)for(int v=1;v<=s;v++) if(f[v][u]>=n){ flag=m-u; u=m+1; break; } printf("%d\n",flag); } return 0; }