const mod = BigInt(10 ** 9 + 7);
const zero = BigInt(0);
const one = BigInt(1);
const ten = BigInt(10);
function substrings(n) {
    const len = BigInt(n.length);
    let sum = zero;
    let f1 = one;
    let f2 = ten;
    for (let i = len - one; i >= zero; i--) {
        [sum, f1, f2] = [
            (sum + BigInt(n[i]) * (i + one) * f1) % mod,
            (f1 + f2) % mod,
            (f2 * ten) % mod
        ];
    }

    return sum;
}