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

#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#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(){
	long z=0;
	long a=0;
	long b=0;
	rd_init();
	for(int c;c=*rp++,c!='\n';++b){
		if(c=='0'){
			z+=b-a>>1;
			a+=1+(a+b&1);
		}
	}
	wt1(z);
	_exit(0);
}