#include<bits/stdc++.h>
using namespace std;
int a[2][20000];
int main()
{
int n;
scanf("%d",&n);
for (int i=1; i<=n; ++i) scanf("%d%d",&a[0][i],&a[1][i]);
int x=1,y=0,ans=0;
while (x<n){
if (a[y][x] || (a[0][x+1] && a[1][x+1])){
ans=-1; break;
}
if (a[y][x+1]) {
y^=1; ++ans; continue;
}
++ans; ++x;
}
if (ans==-1) printf("No");
else printf("%d\n",ans);
return 0;
}