#include<bits/stdc++.h> using namespace std; int dp[101][101]={0}; int main() { int n,m,k,s,a[101],b[101]; while(~scanf("%d%d%d%d",&n,&m,&k,&s)){ for(int i=1;i<=k;k++){ cin>>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++){ dp[v][u] = max(dp[v][u],dp[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(dp[v][u]>=n){ flag = m-u; u=m+1; break; } } } cout<<flag<<endl; } return 0; }