#include void main(void){ int ans = 0 , n = 0; scanf("%d",&n); int x = 0 , y = 0; scanf("%d",&x); scanf("%d",&y); ans = y-x; if(ans < 0){ ans = -1; } for(int i = 0 ; i < n ; i++){ scanf("%d",&x); scanf("%d",&y); } printf("%d",ans); }