week13- A. Become an Escapiest!, Author Solution

#include<cstdio>
int abs(int x){return x<0?-x:x;}
int main(){
    int N,B,P,B_v,P_v;
    scanf("%d%d%d%d%d",&N,&B,&P,&B_v,&P_v);
    for(int i=0;i<N;i++){
        int x;
        scanf("%d",&x);
        if( (P-B)*(x-B)>0              //滿足條件 1
         && P_v*abs(x-B)>B_v*abs(x-P)  //滿足條件 2
        )return 0*puts("useful");
    }
    return 0*puts("useless");
}