#include<iostream> #include<vector> using namespace std; int main(){ int a,n,m,x; cin>>a>>n>>m>>x; vector<long long>up(n+1); for(int i=1;i<=n;++i){ cin>>up[i]; } vector<long long>sum(n+1); sum[1]=a; if(n>=2){ sum[2]=a; } for(int i=3;i<=x;++i){ long long down=up[i-1]; sum[i]=sum[i-1]+up[i]-down; } cout<<sum[x]<<endl; return 0; }