Codeforces Round #207 (Div. 2) A. Group of Students

#include <iostream>
#include <vector>

using namespace std;

int main(){
    int m,sum = 0;
    cin >> m ;
    vector<int> c(m+1,0);
    for(int i = 1; i <= m ; ++ i) {cin >> c[i];sum+=c[i];}
    int x,y,firstPart = 0, secondPart = 0,firstIndex = 0, secondIndex =0;
    cin >>  x >>  y;
    for(firstIndex = 0,secondIndex = m+1;firstIndex < secondIndex;){
        if (firstPart < x) firstPart+=c[++firstIndex];
        if (secondPart < x) secondPart += c[--secondIndex];
        if( firstPart >= x && secondPart >= x ) break;
    }
    if(firstPart > y || secondPart > y) cout<<0<<endl;
    else{
        if(firstIndex >= secondIndex) cout<<0<<endl;
        else{
            if(sum - firstPart - secondPart > 2*(y-x)) cout<<0<<endl;
            else{
                while(secondPart + c[secondIndex-1]<= y && (secondIndex-1) > firstIndex){
                    secondIndex--;
                    secondPart+=c[secondIndex];
                }
                if (secondIndex-1 <= firstIndex) cout<<secondIndex<<endl;
                else{
                    if(sum - secondPart > y) cout<<0<<endl;
                    else cout<<secondIndex<<endl;
                }
            }
        }
    }
}