#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#define rd_skip() while(*rp++>=48)
#define rd() ({long _v=0,_c;while(_c=*rp++-48,_c>=0)_v=_v*10+_c;_v;})
#define wt(v) ({ulong _z=v;do*--wp=_z%10+48;while(_z/=10);})
#define wt1(v) ({char wbuf[64],*wp=wbuf+sizeof wbuf;wt(v);write(1,wp,wbuf+sizeof wbuf-wp);})

typedef unsigned long ulong;

int main(){
	rd_init();
	rd_skip();
	int m=rd();
	rd_skip();
	ulong b=1;
	ulong z=0;
	while(--m){
		ulong a=rd();
		ulong d=a-b;
		z+=d&d-1?:d>>1;
		b=a;
	}
	wt1(z);
	_exit(0);
}