clean way. approaching from two endpoints.

better_298b.cc

/*
 *   updown   /\     up   /    down   \
 *           /  \        /             \
 *          /    \      /               \
 *         v1    v2    v1                v2
 */

    int ans=v1+v2;
    t-=2;
    REP(k,t){
        if(v1<=v2){
            v1+=d;
            ans+=v1;
        }
        else{
            v2+=d;
            ans+=v2;
        }
    }
    cout<<ans<<endl;