#include<stdio.h>
main(){
    int N;
    scanf("%d",&N);
    int x,y;
    scanf("%d %d",&x,&y);
    int ans = y - x;
    for(int i = 1;i < N;i++){
        scanf("%d %d",&x,&y);
        if(ans <= 0 || ans != y-x){
            printf("-1");
            return 0;
        }
    }
    printf("%d",ans);
}