#include    <stdio.h>

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

    scanf("%d", &N);
    for (i = 0; i < N; i++) {
        scanf("%d%d", &X[i], &Y[i]);
    }

    ans = Y[0] - X[0];
    if (ans <= 0) {
        printf("-1\n");
        return 0;
    }

    for (i = 1; i < N; i++) {
        if (ans != (Y[i] - X[i])) {
            ans = -1;
            break;
        }
    }

    printf("%d\n", ans);

    return 0;
}