#include <cstdio>
typedef long long ll;
ll max(ll a,ll b) { return a>b?a:b; }
ll min(ll a,ll b) { return a<b?a:b; }

int main()
{
    int n;
    scanf("%d",&n);
    int i;
    long long curmax,curmin;
    scanf("%lld%lld",&curmin,&curmax);
    long long bef=curmax,cur;
    if(curmin>curmax) goto dead;
    long long newmax, newmin;
    for(i=2;i<n;++i){
        scanf("%lld",&cur);
        if(bef>cur) goto dead;
        newmax=2*bef-curmin;
        newmin=2*bef-curmax;
        curmax = min(newmax,cur);
        curmin = max(newmin,bef);
    }
    printf("%lld\n",max(0,curmax-curmin+1));
    goto done;
    dead:
        puts("0");
    done:
    return 0;
}