#include <cstdio>
#define INF 2000000000
int ans, n, q[100003], v[100003];
void solve(int dist, int c){
    int st=0, dr=0, i;
    for(i=1; i<=n; i++){
        if((st<dr)&&(i-q[st]>dist)){
            st++;
        }
        if((st<dr)&&(ans<v[i]+v[q[st]]-c)){
            ans=v[i]+v[q[st]]-c;
        }
        while((st<dr)&&(v[q[dr-1]]<=v[i])){
            dr--;
        }
        q[dr++]=i;
    }
}
int main(){
    int i;
    scanf("%d", &n);
    for(i=1; i<=n; i++){
        scanf("%d", &v[i]);
    }
    ans=-INF;
    for(i=0; i<20; i++){
        solve((1<<(i+1))-1, i);
    }
    printf("%d\n", ans);
    return 0;
}