#include <stdio.h>

int main(){
    int N, i, ans;
    int X[1000], Y[1000];

    scanf("%d\n", &N);
    scanf("%d %d\n", &X[0], &Y[0]);
    for(i = 1; i < N; i++){
        scanf("%d %d\n", &X[i], &Y[i]);
        if(Y[i] - X[i] > 0){
            if((Y[i] - X[i]) == (Y[i - 1] - X[i - 1])){
                ans = Y[i] - X[i];
                if(i == N - 1){
                    printf("%d\n", ans);
                }
            }else{
                printf("-1\n");
                break;
            }
        }else{
            printf("-1\n");
            break;
        }
    }

    return 0;
}