#!/usr/bin/env python
#coding:utf8

def read():
    N = int(raw_input())
    L = int(raw_input())
    xList = []
    xList.append(0)
    for i in range(N - 1):
        xList.append(int(raw_input()))
    return L, xList


def work((L, xList)):
    xList = xList[::-1]

    ans = 0
    total = 0
    
    for i in range(len(xList)):
        if i > 0:
            # not (xList[i    ] - L / 2 < total / i < xList[i    ] + L / 2) or
            # not (xList[i + 1] - L / 2 < total / i < xList[i + 1] + L / 2)
            if not (i * (2 * xList[i    ] - L)  < 2 * total < i * (2 * xList[i    ] + L)) or \
               not (i * (2 * xList[i - 1] - L)  < 2 * total < i * (2 * xList[i - 1] + L)):
                ans += 1
            
            if abs(xList[i - 1] - xList[i]) >= L:
                print -1
                return
        
        total += xList[i]

    print ans

    
if __name__ == "__main__":
    work(read())