#include<cstdio>

 #include<cstring>

 #define F(i,a,b) for(int i=a;i<=b;i++)

 inline void up(int &x,int y){if(x<y)x=y;}

 int n,m,k,s,a[],b[],dp[][];

 int main(){

     while(~scanf("%d%d%d%d",&n,&m,&k,&s)){

         memset(dp,,sizeof(dp));

         F(i,,k)scanf("%d%d",a+i,b+i);

         F(i,,k)F(j,,s)F(kc,,m-b[i])

         up(dp[j][kc],dp[j-][kc+b[i]]+a[i]);

         int ans=-;

         for(int i=m;i>=;i--)if(dp[s][i]>=n){ans=i;break;}

         printf("%d\n",ans);

     }

     return ;

 }