ll@h,@w,@n,@(a,b)[n],c[],z[],pa,pb,q;
sortA_index(n,b,a,c);
pa=a[0];
pb=b[0];
q=pa-pb;
z[c[0]]=ll_inf;
rep(i,1,n){
	ll y;
	if(b[i]==b[i-1]){
		y=a[i]-a[i-1];
	}else{
		if(b[i]==pb+1){
			y=max(a[i],pa);
		}else{
			y=ll_inf;
		}
		y<?=q+a[i]+b[i]-2;
		pa=a[i];
		pb=b[i];
	}
	z[c[i]]=y;
	q<?=a[i]-b[i];
}
q=a[n-1]+b[n-1];
rrep(i,n-1){
	ll y=z[c[i]];
	if(b[i]==b[i+1]){
		y<?=a[i+1]-a[i];
	}else if(b[i]==b[i+1]-1){
		y<?=max(a[i],a[i+1]);
	}
	y<?=q+a[i]-b[i]-2;
	z[c[i]]=y;
	q<?=a[i]+b[i];
}
wtLn(z(n));